Plus Four trousers were similar in style to knickers or knickerbockers. Why then were they called Plus Fours?

Plus fours are breeches or trousers that extend 4 inches (10 cm) below the knee (and thus four inches longer than traditional knickerbockers, hence the name). As they allow more freedom of movement than knickerbockers, they have been traditionally associated with sporting attire from the 1860s and onward, particularly with golf.

An "extravagant, careless style that fit right in with the looser fashions and lifestyles of the 1920s," plus fours were introduced to America by Edward, Prince of Wales (later Edward VIII), during a diplomatic trip in 1924. They are often seen on golf courses, and frequently worn with argyle socks, silk neckties, and dress shirts or sweaters. Some plus fours came as complete suits. They became associated with bicycle fashion in the 1950s and 60s in Great Britain.

They were later brought back to prominence by the professional golfer Payne Stewart who wore them on the PGA Tour.