A pea-knuckle or thumb war (pea-knuckle or pea-knuckle war in New Zealand) is a game played by two players in a tournament called a thumb-a-war (or thumb war) using the thumbs to simulate fighting. The object of the game is to pin the opponent's thumb, often to a count of three. The San Francisco Chronicle called the game "the miniature golf of martial sports."
The players face each other and each holds out their left hand or right hand in a "thumbs up", and they link hands in a [clarify]. Gameplay has several tactics such as "playing possum", aiming for the knuckle rather than the nail for a pin, going for a quick strike, and waiting for one's opponent to tire. Variations include making the thumbs "bow", "kiss", or both before warring, and to war with both hands at once; or sneak attacks, which involve using your pointer finger to take over the opponent.
The game is typically initiated with both the players uttering the rhyme "One, two, three, four, I declare a thumb war", and their thumbs passing over each other in time with this rhyme. The rhyme is sometimes extended with "Five, six, seven, eight, try to keep your thumb straight." or "Five, six, seven, eight. This should be a piece of cake."  In Spanish, the starting song is "ésta es la pulseada china", with the same thumb dance as in English.
Competitive matches on thumb wrestling have been held on Long Island and at Lowestoft. The 826 Valencia Foundation holds an annual thumb-wrestling competition, which has been won three times by San Francisco Chronicle book editor Oscar Villalon.
Writer Norman Mailer was passionate about thumb wrestling. Author and humorist Paul Davidson claims that his grandfather Bernard Davidson invented the thumb war in the 1940s. American copywriter Julian Koenig claimed to have invented thumb wrestling in 1936 as a boy at Camp Greylock.
A thumb wrestling ring is a toy used for thumb wrestling. The players insert their thumbs in opposite sides and proceed with the thumb war.
- Dieleman, Hans; Huisingh, Don (2006). "The Potentials of Games in Learning and Teaching About Sustainable Development" (PDF). Journal of Cleaner Production: 18. ISSN 0959-6526.
- Villalon, Oscar (8 December 2003). "1, 2, 3, 4, I declare a thumb war". San Francisco Chronicle. Retrieved 2009-10-19.
- Villalon, Oscar (3 December 2006). "THE SEMI-SWEET SCIENCE One, two, three, four. I declare ...". San Francisco Chronicle. Retrieved 2009-10-19.
- Beard, Polly (27 June 2009). "Games to play in your tent on rainy days". The Times. Retrieved 2009-10-19.
- Orr, Elizabeth (22 November 2006). "THUMBS UP!". Columbus Dispatch. Retrieved 2009-10-19.
- Collard, Mark (2005). No props: great games with no equipment. Project Adventure, Inc. p. 218. ISBN 0-934387-05-2.
- Gunter, Veronika Alice; Meyer, Clay (2005). The Ultimate Indoor Games Book: The 200 Best Boredom Busters Ever!. Lark Books. p. 128. ISBN 1-57990-625-7.
- Haslam, Nick (2004). Relational models theory: a contemporary overview. Routledge. p. 374. ISBN 0-8058-5356-1.
- Ketcham, Diane (29 November 1987). "LONG ISLAND JOURNAL". New York Times. Retrieved 2009-10-20.
- "Thumb wars descend on Lowestoft". The Lowestoft Journal. 31 July 2009. Retrieved 2009-10-20.
- Mankiewicz, Josh (26 March 2007). "A real thumbs up for helping kids write well". NBC News. Retrieved 2009-10-20.
- "A Letter From The Publisher". TIME. 16 July 1973. Retrieved 2009-10-20.
- "Who Invented The Game Thumb War". Words for my enjoyment. 18 May 2007. Retrieved 2009-10-20.
- "This American Life: Origin Story". June 9, 2009.
- Tweney, Dylan (30 September 2009). "Pocket Players: 13 Great Portable Games". Wired.com. Retrieved 2009-10-20.
- Villalon, Oscar (2006). The Way Of The Thumb. San Francisco: 826 Valencia. ISBN 0-9790073-1-3.
- Mayer, Andy; Becker, James (January 9, 1983). The Official Book of Thumb Wrestling. Illustrated by Carl Muller. Workman Publishing Company. ISBN 0-89480-363-8.