A round workpiece is placed between the two arms of the centre square, so that its centre can be located.
Notch
The notch in-between the arms of the centre square provides a starting point from which the user can scribe.
Blade
The blade of the centre square bisects the arms with its straight edge. It is used to guide a scriber across the workpiece. The intersection of the lines scribed indicates the location of the centre point.