Parts of a Combination Square – 14 Labeled Parts, 3 Heads

All 14 parts on the diagram

Show the list

Start here: 6 main parts 6

Rule (blade) Hardened steel, graduated, sliding through each head.
Square head (standard head) The head that marks and tests 90 and 45 degrees.
90° face (square face) Held against the work, it sets the rule at a right angle.
45° face (miter face) Held against the work, it sets the rule at 45 degrees.
Spirit level (vial) A bubble vial in the head, so it works as a level.
Graduations Scale marks along the edges; flipping the rule shows others.

The square 4

Scriber Pulls out to scratch lines on metal.
Groove (channel) The lock bolt engages it, so a head clamps anywhere.
Lock nut (knurled nut) Turned, it tightens the head on the rule.
Lock bolt Its end sits in the groove and grips the rule.

Center and protractor heads 4

Center head (centre finder) Its V centres round work so the rule crosses its centre.
Protractor head Sets and reads any angle from 0 to 180 degrees.
Turret Turns on the base and carries the rule round.
Direct-reading scale Degrees 0 to 180, read both ways.

How this list was compiled: Read 2026-09-24. (1) Wikipedia, Combination square, https://en.wikipedia.org/wiki/Combination_square : a rule and interchangeable heads; the heads slide on through a slot and are tightened by a lock bolt or lock nut engaging a channel the full length of the rule; the standard (square) head has faces at 90 and 45 degrees, a small spirit level and a removable scriber; the protractor head: a flat reference edge and an adjustable 180 degree protractor, sometimes called a turret, graduated both ways; the centre finder head has two faces at 90 degrees and the rule's edge bisects them. (2) A square maker's instruction sheet, Introduction, Assembly and Applications, https://info.highlandwoodworking.com/library/manuals/starrett/starrett-combination-square-instructions.pdf : a hardened steel graduated rule (blade) with a machined groove; a square and miter head with a 90 degree square face, a 45 degree miter face, spirit level and scriber; lock bolt and spring loaded lock nut; center head for the centre of round work; protractor head with revolving turrets and a direct reading scale 0 to 180 degrees in opposite directions; reversing the blade shows other graduations. Licence: own work

The drawing: Drawn for this site by script, 2026-09-24. Licence: own work

How it works

A combination square is a steel rule with heads that slide along a groove down its middle and lock anywhere. The square head marks 90 and 45 degrees; a center head and a protractor head can swap in.

Read the full guide

The square head

Its 90° face sets the rule square to an edge and its 45° miter face sets it at a slant. A spirit level sits in the head, and a scriber is stored in its foot. Turning the lock nut drives the lock bolt into the groove.

The other two heads

The center head's V holds round work so the rule's edge crosses the centre. On the protractor head a turret turns on the base, carrying the rule, and the direct-reading scale gives the angle from 0 to 180°.

Questions and answers

What are the parts of a combination square?

A graduated rule with a groove down it, and a square head with a 90° face, a 45° miter face, a spirit level, a scriber, a lock nut and a lock bolt. A set adds a center head and a protractor head with a turret.

What is the blade of a combination square called?

The rule, also called the blade: a hardened steel strip with graduations along its edges and a groove down the middle that the heads clamp into.

What is the pin in a combination square for?

It is the scriber, stored in a hole in the square head. Pulled out, its point scratches a fine line on metal along the rule.