To lock the intelligent cursor to the principal plane

Plane-locking automatically positions any points entered using the mouse to lie on the principal plane of the active workplane.

For example, if the principal plane is XY, each point you enter using the mouse has a Z coordinate of zero. With plane-locking enabled, if you snap to a key point that does not lie on the principal plane

the new position is projected onto the principal plane, along the normal vector of the principle plane:

To enable and disable plane locking

To enter absolute positions

When plane-locking is enabled you can still positions points outside of the principle plane by: