The Morphy number is a measure of how closely a chess player is connected to Paul Morphy (1837–1884) by way of playing chess games. People who played a chess game with Morphy have a Morphy number of 1. Players who did not play Morphy but played someone with a Morphy number of 1 have a Morphy number of 2. People who played someone with a Morphy number of 2 have a Morphy number of 3, et cetera.
The idea is similar to the Erdős number for mathematicians and the Bacon number for actors. For example, Viswanathan Anand, along with many current top players, has a Morphy number of 5: Anand played Efim Geller (Morphy number 4), who played Salo Flohr (Morphy number 3), who played Géza Maróczy (Morphy number 2), who played John Owen (Morphy number 1), who played Morphy.  Taylor Kingston states that the idea of the Morphy number may have originated in a June 2000 note by Tim Krabbé, who has Morphy number 4.
As of September 28, 2010, Leonard Barden, Pal Benko, Arthur Bisguier, Melvin Chernev, Andrija Fuderer, Dennis Horne, Borislav Ivkov, Erik Karklins, Franciscus Kuijpers, Louis Levy, Aleksandar Matanović, Fridrik Olafsson, Jonathan Penrose, Oliver Penrose, Arturo Pomar, Lothar Schmid, and Peter Swinnerton-Dyer are the only known living players with Morphy number 3. Andor Lilienthal and Bent Larsen, who also had Morphy numbers of 3, died in May and September 2010, respectively.
There will probably be few new players with Morphy number 4, although there are probably many thousands of them alive. There may be millions of people with Morphy number 5. Many ordinary players have a Morphy number of 6 (or less).
Morphy number of famous players
These are players who are important in making links for Morphy numbers.
Morphy number 1
Morphy is known to have played about 100 people, but all of the known links for players with Morphy number 2 go through the following five players. Some Irish players could go through the Rev. Dr George Salmon, who played in one of Morphy's blindfold chess simultaneous exhibitions.
Morphy number 2
Everyone in this group played someone in the group above. Some of the most important members of this group follow. Tartakower lost to Mortimer early in his chess career, which spanned beyond WW2, so allowed many younger players to gain a 3.
Morphy number 3
Most of the masters in this group played several members of the previous group. This group includes some of the most important players for making connections to later generations. Botvinnik and Reshevsky played older masters such as Lasker and Janowski, had long careers, and played many younger players. Najdorf was Tartakower's pupil and they played a number of published games together, and Najdorf played blitz right into his 80s, allowing many younger players to achieve 4. C.J.S. Purdy played Tartakower (2), enabling many Australian players to achieve 4. As of January 2013, fewer than six of them are still alive.
Morphy number 4
Morphy number 5
- Your Morphy Number is Up, by Taylor Kingston, PDF
- Playing the Morphy Number Game, by Tim Harding, PDF
- Tim Krabbé - see item #67
- Frederick Rhine, Fun with Morphy Numbers
- Barden's comments to Tim Harding, Playing the Morphy Number Game, chesscafe.com, 2010.
- Edward Winter, Chess Note 6614, June 11, 2010. Retrieved on September 15, 2010.