ANSI B17.1 standard identifies two classes of stock for parallel keys. One is a close, plus toleranced key stock and the other is a broad, negative toleranced bar stock.
Each is combined with appropriate keyseat tolerances to make assemblies. Based on the use of two types of stock, two classes of fit are given.
Class 1 is a clearance or metal-to-metal side fit obtained by using bar stock keys and keyseat tolerances. This is a relatively free fit and applies only to parallel keys.
Class 2 is a side fit, with possible interference or clearance, obtained by using key stock and keyseat tolerances as given in this page. This is a relatively tight fit.
Keyseat: An axially located rectangular groove in a shaft or hub. Hub keyseat has been sometimes referred to as a keyway
Sunk Key: A standard form of key with rectangular or square cross-section in which the key fits into the keyseats on the shaft and the hub.