system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: where does tennis' scoring system come from?

 I kinda get not using units of 1 (e.g. 1, 2, 3...) and instead using an arbitrary count of 15, 30, but then why 40 instead of 45?! 

And also, why not continue adding, 60, 75... instead of going to deuce/advantage?
gpt
One theory is that tennis descended from a French game called  *jeu de paume,* which was similar to tennis but used bare hands instead of a racket.

In  *jeu de paume*, the court was 90 feet in length, with 45 feet on each side.  If a player scored, they moved 15 feet forward; scoring again allowed the player to move another 15 feet forward, but because of the net in the center of the court, scoring a third time only allowed enough room to move forward by 10 feet (thus, 15-30-40).

The popular theory of scoring based on a clock face is considered unlikely, because the first recorded reference to tennis scoring was written in the 15th century, when clocks only measured hours (1-12).  It wasn't until \~1690 that the pendulum escapement was invented and clocks were regularly constructed with minute hands.