If you see a note on a drawing like , check the context.
is the German standard for Thread Undercuts (in German: Gewindefreistich ).
The standard is split into parts, and the variable "$a$" is a critical dimension within them.
Tracking down a clean can feel like a scavenger hunt. Use the official preview for legal accuracy, or rely on standard engineering reference books for the charts. By adhering to these runout and undercut rules, you stop breaking threading tools and stop breaking shafts at the shoulder.
For ISO metric threads (standard bolts and screws).