Coin flipping

Coin flipping, coin tossing, 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. The expression Heads or Tails results from heads and tails being considered opposite body parts.

During a coin toss, the coin is thrown into the air such that it rotates edge-over-edge 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 correctly or was assigned the upper side is declared the winner.

It is possible for a coin to land on its edge, usually by landing up against an object (such as a shoe) or by getting stuck in the ground (as famously happened during the December 8, 2013 NFL match up between the Philadelphia Eagles and Detroit Lions, which took place during a heavy snowstorm). However, even on a flat surface it is possible for a coin to land on its edge, with a chance of about 1 in 6000.Angular momentum typically prevents most coins from landing on their edges unsupported if flipped. Such cases in which a coin does land on its edge are exceptionally rare and in most cases the coin is simply re-flipped.

The coin may be any type as long as it has two distinct sides; it need not be a circulating coin as such. Larger coins tend to be more popular than smaller ones. Most high-profile coin tosses use custom-made ceremonial medallions.

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.

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Wikipedia