Centre squares are available in a number of sizes.
The size of a centre square is determined by the length of its blade. The blades of centre squares are usually either 38mm (11/2“) or 75mm (261⁄64“) long. Larger squares do exist but are harder to come across.
The length of the blade of a centre square is related to the capacity of the tool. The capacity of a centre square determines the maximum diameter an object can have if it is to be used with the tool.
