When was the yellow jersey introduced in Tour de France?

Explanation

The yellow jersey (in French: Maillot Jaune) is worn by the current leader of the Tour de France. The jersey was introduced in the middle of the race in 1919, to make it easier to spot the frontrunner. The jersey is worn by the rider, who has spent the shortest amount of time overall on the stages, plus any time bonuses. The first three to finish a stage get 20, 12 and 8 seconds deducted respectively. The first three in the sprints can deduct 6,4 and 2 seconds.