The “square centre” is a very useful chuck when using square or even rectangular stock. It is a versatile chuck which is a no prep chucking method for setup between centres.
The taper of the sides in the square pyramid allows for the accommodation of any size up to the base length of the square
The chuck can be used in instances where you may use a spur centre or a cup chuck with tailstock support.
I often use this square centre when preparing the tailstock end of the work to fit a cupchuck.