Knickerbockers or knee breeches are pants that only go down to the knee and were quite popular in the first half of the twentieth century.
Bobby Jones, among other golfers, found knickerbockers and breeches too restrictive for a full swing.
Tailors solved this by designing special golf knickers with an additional four inches below the knee seam, calling them “plus fours.”
The extra length allowed just enough slack to free up the golfer’s swing.
Some players still wear them to this day.
