Why does tennis score 15, 30, 40?

The leading explanation traces the sequence to medieval French tennis, where a clock face marked the score, moving a hand a quarter turn per point: 15, 30, 45, game. Over time 45 was compressed in speech to 40, which is easier to say quickly. The theory is plausible rather than proven, since documentation from the period is thin, and love for zero probably comes from l'oeuf, the French word for egg, describing the shape of a zero, though some scholars argue for the sense of playing for love, meaning for nothing.

The structure that follows is more useful to know than the etymology. Four points win a game, but you must win by two, so at 40-40 the score is deuce and play continues with advantage until someone takes two consecutive points. Six games win a set, again by two, with a tiebreak usually played at 6-6 to prevent sets running indefinitely. Matches are best of three sets, or best of five for men at the Grand Slams. The win-by-two requirement at every level is what produces the sport's famous marathons, most notoriously the 2010 Wimbledon match between Isner and Mahut that reached 70-68 in the final set, after which the majors adopted final-set tiebreaks.