The Centring Head holds the blade at a 45° angle relative to machined 90° surfaces. Marking the two diagonals of a square locates centre. It’s even easier to find the centre of a circle…just hold the Centring Head against the circle in any two different spots and strike a line across the diameter. The intersection is the dead-centre of your circle. Opposite the 90° centring guides, the back side of the Centring Head locks in with the indexing feature so you can scribe without changing heads.