Fasteners such as screws, bolts and nuts have threads that allow two objects to be assembled and disassembled. The size of the threads are indicated on a drawing using a leader and a note or "CALL OFF". The thread specification contains:

• the nominal diameter of the fastener
• the number of threads per inch - TPI
• the thread Series - UNC (Unified National Course), UNF (Unified National Fine) or UNEF (Unified National Extra Fine)
• the class of Fit - 1 (loose), 2 (average) or 3 (tight)
• the type of thread - A (external) or B (internal)
• the length of the fastener

• the letter M - to denote metric system
• the diameter in millimetre
• the Pitch in millimetre - distance between crest of threads
• the thread tolerance (class of Fit)

1. (free) 5H, internal & 4g, external
2. (medium) 6H, internal & 6g, external
3. (tight) 7H, internal & 8g, external