Explain the history of the scoring system in lawn tennis.?

Question

Why is the scoring for instance 15-0, 30-0, 40-0 and not say 45-0? What is the genesis of such a scoring system?

Answer 1

Originally, the French who played tennis originally played for money.  The units of money to be played for were 15. 30, and 40.  I'm sure there are more details surrounding this scoring system, but this is the basic origin.

Answer 2

the system may be based on the presence of a clock face at the end of the tennis court. A quarter move of the appropriate hand was made after each rest, with the score being called as 15, 30, or 45. As the hand was moved to 60, this was the game. This didn't explain a score of 40, however, so we continued on.

and

Tennis scoring has its origin in medieval numerology. The number 60 was considered to be a "good" or "complete" number back then, in about the same way you'd consider 100 to be a nice round figure today. The medieval version of tennis, therefore, was based on 60 -- the four points when 15, 30, 45 (which we abbreviate to 40) and 60, or game.