To win a game in tennis, a player must win four points and must win by a margin of two. The scoring goes as follows, after starting at love: 15, 30, 40, game.
This system is medieval in origin. It never has been satisfactorily explained why three points equal 40 rather than 45. Zero is generally referred to as “love,” which is thought to be derived from l’oeuf, the French word for “egg.” The server’s score is called first; thus, 30–15 means that the server has two points to one, whereas 15–30 means that the receiver has two points to one. If both players reach 40, the score is said to be “deuce,” and the game continues until a player achieves first “advantage” and then the two-point margin for “game.” There is no limit to the number of times a game can go to deuce before it is decided, but in some competitions a so-called “no-ad” system is used, which means that no two-point margin is required and the first player to win four points wins the game.
As points make up a game, games make up a set, and sets make up a match. The first player to win six games traditionally wins the set, although a two-game margin is again required; thus a set in which each player has won five games cannot be won before 7–5. So as to prevent matches from going on (seemingly) forever, though, tiebreakers that eliminate the need to win a set by two games when both players have won six games are typically used in competitions. That means that a set can end 7–6.