Parallel Keys are the most widely used. They have a square or rectangular cross-section. Square keys are used for smaller shafts and rectangular faced keys are used for shaft diameters over 6.5 in (170 mm) or when the wall thickness of the mating hub is an issue.
The cross section of the key is either square or rectangular and hence the name. This is the most standardized version of keys currently being used in all applications. The size of the key to be selected based on the shaft size is already been standardized for rectangular keys.
This article about Shaft Keys describes the types of keys and the design or selection procedures to be adopted.Shafts are indispensable mechanical power transmission elements that are mainly used in for transmitting rotary motion and torque from one point to another point.
The Woodruff Keys is so called because of its manufacturing method. It consists of a semicircular key seat on the shaft surface which holds the key. The key is in the shape of a section of a circle.