Undercut (turning)

On turned parts an undercut is also known as a neck or "relief groove".

[1] Strictly speaking the relief simply needs to be equal or slightly smaller than the minor diameter of the thread.

They are also often used on shafts that have diameter changes so that a mating part can seat against the shoulder.

If an undercut is not provided there is always a small radius left behind even if a sharp corner is intended.

These types of undercuts are called out on technical drawings by saying the width and either the depth or the diameter of the bottom of the neck.