Theory 1: The French used 2 clocks to keep game scores of each player. Since 4 points were needed to win a game, when a player won a point, the hands of the clock were advanced by a quarter. So it went from 0 to 15, 15 to 30. Logically it should have been 0-15-30-45-60(game). However, since a game needs to be won by difference of two points, the French choose stops at 40-50-game. If any player failed to win 2 points in a row, the clock would move back to 40.
Theory 2: The clock was the reason for choosing 0-15-30 as the first 3 points. However, in French 0-15-30-45 would be zero – quinze – trente – quarante cinq. Since ‘quarante cinq’ is 3 syllables long (and difficult for the English to pronounce!), 40 (quarante) was chosen instead.
Theory 3: The scoring nomenclature could have come from the French game jeu de paume (a game which used the hand instead of a racket). In Jeu de paume the traditional court was 90 ft in total with 45 ft on each side. When the server scored, he moved forward 15 ft. If he scored again, he would move another 15 ft (30 in all). If he scored a third time, he could only move 10 ft closer (40 in all).