From Wikipedia: 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, and are particularly associated with golf.
Less known are plus twos, plus sixes and plus-eights, of similar definitions.
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 of the United Kingdom), during a diplomatic trip in 1924. They are often seen on golf courses, and frequently worn with argyle socks, silk neckties, and dress shirts/sweaters. Some plus fours even came as complete suits.
Check out these great examples:
This Saturday we'll have the Tweed Outfitter in our Tweed pop up shop - turning your old pants into Plus Fours for the Tweed Ride. Click here for more information.
See more Tweed Ride posts by Rocket Fuel here
