Coin flipping, coin tossing, cross and pile, or heads or tails is the practice of throwing a coin in the air to choose between two alternatives sometimes to resolve a dispute between two parties. It is a form of sortition which inherently has only two possible and equally likely outcomes.
The historical origin of coin flipping is the interpretation of a chance outcome as the expression of divine will.
Coin flipping as a game was known to the Romans as navia aut caput ("ship or head"), as some coins had a ship on one side and the head of the emperor on the other. In England, this game was referred to as cross and pile.
During coin flipping the coin is tossed into the air such that it rotates end-over-end several times. Either beforehand or when the coin is in the air, an interested party calls "heads" or "tails", indicating which side of the coin that party is choosing. The other party is assigned the opposite side. Depending on custom, the coin may be caught, caught and inverted, or allowed to land on the ground. When the coin comes to rest, the toss is complete and the party who called or was assigned the face-up side is declared the winner. If the outcome is unclear the toss is repeated; for example the coin may come to rest upright against another object or fall down a drain.
The coin may be any type as long as it has two distinct sides; it need not be a coin as such.
Fraudulent flipping 
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It is not very difficult to learn to flip a coin so as to get a reliable intended result, not by controlling the number of flips but by creating the illusion that the coin is flipping. The coin remains at a constant inclination to the vertical and simply rotates, or wobbles, about a vertical axis. The inclination must be sufficient for the coin to occupy most of the sphere that a fairly flipped coin would, while not being so great that the coin is likely to bounce when caught. An inclination around 45 degrees is usually satisfactory.
Another simple way to cheat is simply to peek at the coin as it lands in your hand. Although it seems that this would be easily detectable, in fact, this can be done quickly and convincingly with some practice.
The third common method of fraudulent flipping is to determine which side is up by the feel of the coin. On most USA coins, the heads side is smoother than the tails side.
Three-way coin flips are also possible, by a different process – this can be done either to choose two out of three, or to choose one out of three. To choose two out of three, three coins are flipped, and if two coins come up the same and one different, the different one loses (is out), leaving two players. To choose one out of three, either reverse this (the odd coin out is the winner), or add a regular two-way coin flip between the remaining players as a second step. Note that the three-way flip is 75% likely to work each time it is tried (if all coins are heads or all are tails, which occurs 1/4 of the time, the flip is repeated until the results differ), and does not require that "heads" or "tails" be called.
A famous example of such a three-way coin flip (choose two out of three) is dramatized in Friday Night Lights (originally a book, subsequently film and TV series), where three high school football teams with identical records use a three-way coin flip – at a truck stop – to determine which two will advance to the playoffs. A legacy of this coin flip was to reduce the use of coin flips to break ties in Texas sports, instead using point-systems to reduce the frequency of ties.
Use in dispute resolution 
Coin tossing is a simple and unbiased way of settling a dispute or deciding between two or more arbitrary options. In a game theoretic analysis it provides even odds to both sides involved, requiring little effort and preventing the dispute from escalating into a struggle. It is used widely in sports and other games to decide arbitrary factors such as which side of the field a team will play from, or which side will attack or defend initially; these decisions may tend to favor one side, or may be neutral. Factors such as wind direction, the position of the sun, and other conditions may affect the decision. In team sports it is often the captain who makes the call, while the umpire or referee usually oversees such proceedings. A competitive method may be used instead of a toss in some situations, for example in basketball the jump ball is employed, while the face-off plays a similar role in ice hockey.
Coin flipping is used to decide which end of the field the teams will play to and/or which team gets first use of the ball, or similar questions in football matches, American football games, Australian rules football, volleyball, and other sports requiring such decisions. In the U.S. a specially minted coin is flipped in National Football League games; the coin is then sent to the Pro Football Hall of Fame, and other coins of the special series minted at the same time are sold to collectors. The XFL, a short-lived American football league, attempted to avoid coin tosses by implementing a face-off style "opening scramble," in which one player from each team tried to recover a loose football; the team whose player recovered the ball got first choice. Because of the high rate of injury in these events, it has not achieved mainstream popularity in any football league, and coin tossing remains the method of choice in American football.
In a football match, the team winning the coin toss chooses which goal to attack in the first half; the opposing team kicks off for the first half. For the second half, the teams switch ends, and the team that won the coin toss kicks off. Coin tosses are also used to decide which team has the pick of going first or second in a penalty shoot-out. Before the introduction of the penalty shoot-out, coin tosses were occasionally needed to decide the outcome of tied matches. The most famous instance of this was the 1968 European Football Championship semi-final between Italy and the Soviet Union, which finished 0-0 after extra time. Italy won, and went on to become European champions.
In duels a coin toss was sometimes used to determine which combatant had the sun at his back. In some other sports, the result of the toss is less crucial and merely a way to fairly choose between two more or less equal options.
The National Football League also has a coin toss for tie-breaking among teams for playoff berths and seeding, but the rules make the need for coin toss, which is random rather than competitive, very unlikely. A similar procedure breaks ties for the purposes of seeding in the NFL Draft; these coin tosses are more common, since the tie-breaking procedure for the draft is much less elaborate than the one used for playoff seeding.
Major League Baseball once conducted a series of coin flips as a contingency on the last month of its regular season to determine home teams for any potential one-game playoff games that might need to be added to the regular season. Most of these cases did not occur. From the 2009 season, the method to determine home-field advantage was changed.
Fédération Internationale d'Escrime rules use a coin toss to determine the winner of a fencing match that remains tied at the end of a "sudden death" extra minute of competition. Although in most international matches this is now done electronically by the scoring apparatus.
In the United States Asa Lovejoy and Francis W. Pettygrove, who owned the claim to the land that would later become Portland, Oregon, each wanted to name the new town after their respective hometowns of Boston, Massachusetts and Portland, Maine; Pettygrove won the coin flip.
In December 2006 Australian television networks Seven and Ten, which shared the broadcasting of the 2007 AFL Season, decided who would broadcast the Grand Final with the toss of a coin. Network Ten won.
In some jurisdictions, a coin is flipped to decide between two candidates who poll equal number of votes in an election, or two companies tendering equal prices for a project. For example, a coin toss decided a City of Toronto tender in 2003 for painting lines on 1,605 km of city streets: the bids were $161,110.00 ($100.3800623 per km), $146,584.65 ($91.33 per km, exactly), and two equal bids of $111,242.55 ($69.31 per km, exactly). The numerical coincidence is somewhat less remarkable than it appears, because three of the four bids are for a whole number of cents per kilometer.
"Drawing of lots" is one of the methods to break ties to determine a winner in an election; the coin flip is considered an acceptable variant. Each candidate will be given five chances to flip a coin; the candidate with the most number of "heads" wins. The 2013 mayoral election in San Teodoro, Oriental Mindoro was decided on a coin flip, with a winner being proclaimed after the second round when both candidates remained tied in the first round.
United Kingdom 
In the United Kingdom, if a local or national election has resulted in a tie where candidates receive exactly the same number of votes, then the winner can be decided either by drawing straws/lots, coin flip, or drawing a high card in pack of cards.
Experimental and theoretical analysis of coin tossing has shown that the outcome is predictable, to some degree at least, if the initial conditions of the toss (position, velocity and angular momentum) are known. Coin tossing may be modeled as a problem in Lagrangian mechanics. The important aspects are the tumbling motion of the coin, the precession (wobbling) of its axis, and whether the coin bounces at the end of its trajectory.
The outcome of coin flipping has been studied by Persi Diaconis and his collaborators. They have demonstrated that a mechanical coin flipper which imparts the same initial conditions for every toss has a highly predictable outcome — the phase space is fairly regular. Further, in actual flipping, people exhibit slight bias – "coin tossing is fair to two decimals but not to three. That is, typical flips show biases such as .495 or .503."
In studying coin flipping, to observe the rotation speed of coin flips, Diaconis first used a strobe light and a coin with one side painted black, the other white, so that when the speed of the strobe flash equaled the rotation rate of the coin, it would appear to always show the same side. This proved difficult to use, and rotation rate was more accurately computed by attaching floss to a coin, such that it would wind around the coin – after a flip, one could count rotations by unwinding the floss, and then compute rotation rate as flips over air time.
Moreover, their theoretical analysis of the physics of coin tosses predicts a slight bias for a caught coin to be caught the same way up as it was thrown, with a probability of around 0.51, though a subsequent attempt to verify this experimentally gave ambiguous results. Stage magicians and gamblers, with practice, are able to greatly increase this bias, whilst still making throws which are visually indistinguishable from normal throws.
Since the images on the two sides of actual coins are made of raised metal, the toss is likely to slightly favor one face or the other if the coin is allowed to roll on one edge upon landing. Coin spinning is much more likely to be biased than flipping, and conjurers trim the edges of coins so that when spun they usually land on a particular face.
Counterintuitive properties 
Human intuition about conditional probability is often very poor and can give rise to some seemingly surprising observations. For example, if the successive tosses of a coin are recorded as a string of "H" and "T", then for any trial of tosses, it is twice as likely that the triplet TTH will occur before THT than after it. It is three times as likely that THH will precede HHT. (See Penney's game)
The mathematical abstraction of the statistics of coin flipping is described by means of the Bernoulli process; a single flip of a coin is a Bernoulli trial. In the study of statistics, coin-flipping plays the role of being an introductory example of the complexities of statistics. A commonly treated textbook topic is that of checking if a coin is fair.
Coin flipping in telecommunications 
There is no reliable way to use a true coin flip to settle a dispute between two parties if they cannot both see the coin—for example, over the phone. The flipping party could easily lie about the outcome of the toss. In telecommunications and cryptography, the following algorithm can be used:
- Alice and Bob each choose a random string, "ljngjkrjgnfdudiudd" and "gfdgdfjkherfsfsd" respectively.
- Alice chooses an outcome for an imaginary coin flip, such as "tail"
- Bob sends Alice his random string "gfdgdfjkherfsfsd"
- Alice immediately computes a SHA-1 hash of the string "tail ljngjkrjgnfdudiudd gfdgdfjkherfsfsd", which is 59dea408d43183a3937957e71a4bcacc616d9cbc and sends it to Bob
- Alice asks Bob: "heads or tails"?
- Bob says, for instance, "heads".
- Alice tells him she's just won, and proves it by showing the string "tail ljngjkrjgnfdudiudd gfdgdfjkherfsfsd".
- Bob can check that Alice didn't lie by computing the SHA-1 of the string himself
- Furthermore Bob by providing his own randomly generated string guarantees that Alice wasn't able to precompute an image pair of "tail/random string" or "head/random string".
In lotteries 
The New Zealand lottery game Big Wednesday uses a coin toss. If a player matches all 6 of their numbers, the coin toss will decide whether they win a cash jackpot (minimum of NZ$25,000) or a bigger jackpot with luxury prizes (minimum of NZ$2 million cash, plus value of luxury prizes.) The coin toss is also used in determining the Second Chance winner's prize.
Use in clarifying feelings 
A technique attributed to Sigmund Freud to help in making difficult decisions is to toss a coin not actually to determine the decision, but to clarify the decision-maker's feelings. He explained: "I did not say you should follow blindly what the coin tells you. What I want you to do is to note what the coin indicates. Then look into your own reactions. Ask yourself: Am I pleased? Am I disappointed? That will help you to recognize how you really feel about the matter, deep down inside. With that as a basis, you'll then be ready to make up your mind and come to the right decision."
In fiction 
Coin Toss(2012) an American independent film set in Chicago and Directed by Satya Kharkar. In this movie character uncle Turley an eccentric mathematics genius keeps on flipping coin to decide upon his daily tasks like whether to go for a walk or not.
George Raft became famous as the coin-flipping gangster "Guino Rinaldo" in the 1932 Howard Hawks/Howard Hughes film Scarface (1932). Bugs Bunny parodies Raft in the classic 1946 animated short film Racketeer Rabbit. Raft himself later parodied his own gangster persona as the character "Spats Colombo" in Billy Wilder's 1959 comedy Some Like It Hot: Raft sees another mobster flipping a coin and responds, "Where did you pick up that cheap trick?" Raft's coin-tossing established a distinctive motif used in numerous later gangster movies.
In the climax of Sholay, Veeru and Jaidev decide their next strategy over their encounter with the villains by tossing a coin (they are in habit of deciding over the affairs between themselves this way). It is revealed at the end that the coin used by him is actually a trick coin that always come up heads.
The 1972 movie adaptation of Graham Greene's novel Travels with My Aunt ends with a coin toss that will decide the future of one of the characters. The movie ends with the coin in mid-air, leaving their fate unresolved.
The DC Comics supervillain Two-Face, (most famously as a member of Batman's rogues gallery), has a double-headed coin with one side defaced—a parallel to his actual character, because one side of his face is deformed from acid throwing—which he relies upon for all of his decisions. He will do evil if it lands on the defaced side, and good on the other side. The coin is also representative of alter-ego Harvey Dent's obsession with dualism and the number 2. In the film The Dark Knight, the coin starts out clean, and Harvey Dent (played by Aaron Eckhart) uses this trick coin to seemingly leave important decisions to chance ("Heads I go through with it"). The coin is later blackened on one side in the explosion that kills his fiancée, Rachel Dawes. In Batman Forever, Two Face, (portrayed by Tommy Lee Jones), flips his coin multiple times in one scene to see how far Bruce Wayne can come within range of his gun. After a number of tries, the scarred side finally comes up and he fires anyway.
In the video game Final Fantasy VI brothers Edgar and Sabin flip a coin in order to determine who succeeds the throne of Figaro. It is later revealed that Edgar used a double-headed coin in order to win, allowing Sabin to live without the burden of the kingdom. This coin is also seen if Edgar is present in the first encounter with the gambler Setzer, who is highly amused by it when it is used to trick him into providing his airship.
In the video game "Shenmue 2" gang leader Wuying Ren carries a double-sided coin in each pocket, asking for a head or tail call before pulling the coin out and flipping the coin. This process guarantees him victory in the outcome of the coin toss, usually forcing protagonist Ryo into a dangerous situation. The trickery behind this method is revealed as the characters part ways at the end of the game.
In Futurama episode The Farnsworth Parabox, Professor Farnsworth creates a parallel universe. The only difference between our universe and the other is that every time someone flipped a coin, it landed on the opposite side. This leads to extremely different worlds and humorous confusion.
The DVD of Final Destination 3 has a special feature allowing the viewer to flip a coin apparently to determine the outcome of the movie; however, the outcome is fixed to maintain the plot, and the coin flip is ignored.
The final episode of the American television series JAG ends with an incomplete coin flip.
In the book No Country for Old Men (and the film made of it), Anton Chigurh, the story's villain, occasionally flips coins for potential victims to decide whether or not to kill them. He allows people to place their life in the hands of divine providence, and those who refuse to choose are killed anyway, for their obstinacy and refusal to submit to Fate. The meaning of Chigurh's coin-flipping is left ambiguous (in both the book and the film), and has led to considerable discussion: commentators suggest, for example, that Chigurh views himself as simply following the will of the universe, or is "merely cruel," or that it is an inevitable outgrowth of his (perceived) atheism or that Chigurh is in fact a stand-in for fate, or alternatively that his adherence to chance is a way for him to deny responsibility for his actions or to displace that responsibility onto his victims.
In the manga/anime of Hunter x Hunter by Yoshihiro Togashi, a servant of the Zaolydeck family challenges Gon and his companions, Leorio and Kurapica, to a game involving a coin flip. The game is simple: Yoshihiro flips the coin in the air and quickly snatches it before the coin falls, then Gon or his companions have to say which hand the employee caught the coin with. This proves to be incredibly difficult due to the unrealistic speed of the coin flipper's hands. Gon is very observant and is occasionally able to guess right. See Flipism.
In The Mentalist episode "Blood In, Blood Out" during season 2, CBI consultant Patrick Jane wins a wager by flipping a coin and it landing on heads 20 times in a row. It is later shown that he rigged the coin in his favor.
In the video game Bioshock Infinite there is a coin-flipping sequence early in the game. Booker DeWitt is asked by the Lutece Twins to choose a side. He sometimes chooses "heads" but other times "tails" - but the coin always lands on heads. This is tightly connected to a central subject of the narrative (constants and variables in different realities) and to a central thematic of the game (the illusion of one's choices having an impact).
Coin landing on its edge in fiction 
A coin toss has a theoretical third outcome, in which the coin comes to rest upright on its edge, rather than falling to either heads or tails. Such an outcome is nearly impossible in reality, but is seen in fiction, often for comedic effect. Such an outcome usually results in either a tied coin toss, or victory to a person who successfully called "edge".
In the 1939 film Mr. Smith Goes to Washington, a state governor has to select an interim Senator, and he is being pressured by two opposing factions to choose between their respective candidates, Mr. Hill and Mr. Miller. Unable to choose, he flips a coin in the privacy of his office, but it falls against a book and lands on edge. Consequently, he makes neither choice and chooses Mr. Smith.
In The Twilight Zone episode "A Penny for Your Thoughts," the main character buys a newspaper, and flips a coin into the collection pan, where it lands on its edge. As a consequence, he can hear people's thoughts, but at the end of the day he knocks the coin off its edge when dropping another coin into the pan, which causes him to lose his telepathic ability.
In the American comedy film Mouse Hunt, out-of-work brothers Lars and Ernie toss a coin to decide who gets to sleep in the only bed in the inherited house. The coin ends up spinning on the floor and coming to rest on edge, so the brothers share the bed.
The Hong Kong-made film Shaolin Soccer contains a scene in which one of Sing's brothers is being asked to join Sing's soccer team, and he refuses because he mathematically predicts the team will fail; he uses a coin toss to demonstrate his point, saying it has zero chance of landing on its edge. When the coin is carelessly dropped later in the scene, the brother is amazed to discover that it has, indeed, landed on its edge and become stuck inside a small crack in the asphalt.
In an episode of Malcolm in the Middle, Malcolm decides to flip a coin in order to resolve a dispute about keeping a potentially offensive cardboard cut-out up in the store that he works in (citing that logic wasn't good enough). The coin is shown to land on its edge, leaving Malcolm uncertain what to do.
In Scrubs episode "My Best Friend's Baby's Baby and My Baby's Baby", protagonist J.D. and Kim cannot decide whether or not to keep their baby after an accidental pregnancy. When all else fails, they flip a coin, which lands on its edge.
In the video game Soul Reaver 2, Kain proposes the possibility of a coin landing on its edge when discussing a fateful outcome critical to plot development.
See also 
- Bernoulli process
- Checking if a coin is fair
- Penney's game
- Gambler's fallacy
- Rock paper scissors
- Alleyne, Richard (December 31, 2009). "Coin tossing through the ages". The Telegraph. Retrieved 2012-12-08.
- "Cross and Pile". Dictionary of Phrase and Fable. Bartleby.com. 1898. Retrieved 2012-12-08.
- Bissinger, H. G. Bissinger (1990). "Chapter 13: Heads or Tails". Friday Night Lights: A Town, a Team, and a Dream. Da Capo Press. ISBN 9780306809903. Retrieved 2012-12-08. The three teams are Permian, Midland Lee, and Midland High – which lost the toss. This was the 1988 season, and the three schools had identical 5–1 district records; overall records differed.
- Lee, Mike (November 7, 2008). "SAISD athletic director looks back on 1988's famous coin-flip". San Angelo Standard-Times. Retrieved 2012-12-08.
- "French Duels". Scribner's Monthly 11: 546. 1876. Reprinted in "French Duels" (PDF). The New York Times. January 23, 1876.
- "Ownership approves two major rules amendments" (Press release). Major League Baseball. January 15, 2009. Retrieved 2012-12-08.
- Orloff, Chet. "Francis Pettygrove (1812–1887)". The Oregon Encyclopedia. Portland State University. Retrieved March 29, 2010.
- Example: Meredith, R. W.; Janecka, J. E.; Gatesy, J.; Ryder, O. A.; Fisher, C. A.; Teeling, E. C.; Goodbla, A.; Eizirik, E. et al. (2011). "Impacts of the Cretaceous Terrestrial Revolution and KPg Extinction on Mammal Diversification". Science 334 (6055): 521–4. doi:10.1126/science.1211028. PMID 21940861. "First authorship determined by coin toss. [...] Last authorship determined by coin toss."
- Virola, Madonna (2013-05-16). "Coin toss breaks tie in mayoral race in Oriental Mindoro town". Philippine Daily Inquirer. Retrieved 2013-03-16.
- "Hague savours local victories". BBC News. May 5, 2000. Retrieved 2012-12-08. "There are two methods to decide the outcome in the event of a draw - either a coin is flipped or the parties draw straws."
- "The count". Vote2001 (BBC News). 17 February 17, 2001. Retrieved 2012-12-08. "He or she [the returning officer] can use any random method such as tossing a coin, but the recommended way is to ask each candidate to write their name on a blank slip of paper and place it in a container."
- Diaconis, Persi (11 December 2002). "The Problem of Thinking Too Much". Department of Statistics, Stanford University.
- Landhuis, Esther (June 7, 2004). "Lifelong debunker takes on arbiter of neutral choices". Stanford Report.
- Aldous, David. "40,000 coin tosses yield ambiguous evidence for dynamical bias". Department of Statistics, University of California, Berkeley.
- "Coin Tossing". Wolfram MathWorld.
- Mackay, Harvey (28 May 2009). "Decision making defines the leader". Archived from the original on 24 July 2011.
- Bourne, Mark (2006). "Some Like It Hot: Collector's Edition". The DVD Journal.
- Rutter, Ben (June 15, 2005). "No Country for Old Dudes". n+1.
- Emerson, Jim (March 28, 2008). "No God for Anton Chigurh?". Scanners. Chicago Sun-Times.
||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (December 2010)|
- Ford, Joseph (1983). "How random is a coin toss?". Physics Today 36 (4): 40–47. doi:10.1063/1.2915570.
- Keller, Joseph B. (1986). "The probability of heads". American Mathematical Monthly (Mathematical Association of America) 93 (3): 191–197. doi:10.2307/2323340. JSTOR 2323340.
- Vulovic, Vladimir Z.; Prange, Richard E. (1986). "Randomness of a true coin toss". Physical Review A 33 (1): 576–582. doi:10.1103/PhysRevA.33.576. PMID 9896645.
|Wikimedia Commons has media related to: Coin flipping|
- Heads or Tails? (A discussion of the predictability of a coin toss; with references)
- The Not So Random Coin Toss (Brief blurb about Persi Diaconis' work, with a photograph of the coin-tossing machine)
- Dynamical Bias in the Coin Toss (by Persi Diaconis, Susan Holmes, and Richard Montgomery; very detailed)
- Whether divination by drawing lots is unlawful? (From the Summa Theologica of Thomas Aquinas)
- The Casting of Lots (Discussion of making decisions by chance outcomes throughout history)
- Coin Tossing — mathworld.com (Contains information about counterintuitive properties of coin tossing)
- Leads in Coin Tossing by Fiona Maclachlan, The Wolfram Demonstrations Project.