Knickerbockers, later shortened to “knickers”, were popular casual wear for the well-dressed gentleman. Variations of knickers included plus-fours, plus- sixes, plus-eights and plus-tens. The “plus” in the term referred to how many inches below the knee they hung. See AdrianaSassoon.me - MEN’S FASHION 1920.
First introduced during the 1920s, plus fours were a variation on the traditional knee pants called knickers, which had been worn by men, boys, and, occasionally, women, since the late 1800s. Plus fours received their name because they were made four inches longer than ordinary knickers.
Plus twos and and plus sixes were also introduced but did not become as popular as the plus fours.
Plus twos, with an extra 2 inches of material to fold over the knee, used less fabric than plus fours and often better suit the taller and slimmer chap.
Plus sixes, which used more, six inches below the knee, are fuller but much less common.
How many different kinds of pants exist? You probably know that there are many different types of pants. Refer to ApparelSearch.com's Types of Pants Defined for the Fashion Industry to learn more.