Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of thread relief (undercut). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
Without a relief groove, a threading tool has nowhere to stop — it either runs into a shoulder and breaks, or the thread fades out before reaching it and the nut cannot seat against the face.
A relief groove also creates a stress concentration, so on fatigue-critical parts a rounded runout is preferred over a square groove. The trade-off is that a rounded runout does not let the nut seat, so it is only usable where the thread does not need to bottom against a face.
Common misuse
Omitting the relief and expecting the nut to seat against the shoulder. The thread cannot be cut all the way to a square corner.