To complicate things, ½ and ¼ sizes were also introduced. For instance, a No.5½ jack plane is a slightly wider, heavier version of the No.5, while the No. 5¼ is a narrower, lighter version of the No. 5.
Some planes have their model numbers on the toe or front end of the plane. Sometimes the number is on the heel or another part of the plane’s body, and some types don’t bear the number at all!