What a square does
A square carries one angle — 90°, and on some squares 45° — from a reference surface to a line. Its stock, head or lip registers on a face or edge you trust, and its blade gives the line. That makes a square only as good as two things: its own accuracy, and the reference it is held against. The second is covered in how to measure and mark accurately; this page is about the first.
The main kinds
Carpentry squares differ less in precision than in what they are built to do (see the plate above).
| Square | Blade | Adjustable? | Common job |
|---|---|---|---|
| Try square | Fixed steel blade riveted into a stock — Crown’s 9 in model has a rosewood stock with brass fittings | No | Squaring lines across a board and checking edges at the bench |
| Combination square | Ruled steel blade in a sliding head with 90° and 45° faces | Yes: the head slides and locks | Squaring and mitre lines, setting and repeating distances, depth checks |
| Engineer’s square | Hardened, ground steel; usually no scale | No | Checking other squares, machine set-ups and small work where accuracy matters most |
| Speed square | A triangle with a lip; Swanson’s is 7 in, die-cast aluminium | No | Quick 90° and 45° lines and saw guidance; angles from its pivot |
| Framing square | Two long blades, 16 × 24 in on Swanson’s | No | Large work: squaring frames and panels, stair and rafter layout |
Makers’ descriptions of their own squares. Swanson describes its Speed Square as a try square, mitre square, saw guide, line scriber and protractor in one.
Fixed or adjustable
A fixed square has one joint between blade and stock, set when it is made. An adjustable square — the combination square — has a head that slides, and has to lock back at 90° every time. Starrett lists what that buys you: depth and height gauging, a marking gauge, a level and a straightedge as well as 90° and 45°. The cost is that dirt under the head, or a lock that creeps, puts error into the setting. Starrett contrasts its cast-iron and forged, hardened steel heads with the plastic and die-cast heads on cheaper squares. The pilot explains using a combination square as a gauge.
Blade length
A longer blade squares a wider board in one go, and — because the same angle error grows with distance — makes an out-of-square blade easier to see. A shorter one is lighter and handier for joinery. Many workshops keep a short square for joint layout and a long one, or a framing square, for panels and carcasses.
What makers say about accuracy
Makers state accuracy in different forms — per inch, per 6 in, over a stated length — and some state none at all. Quote a figure only with its length, and compare two squares only over the same length.
| Square | Stated tolerance | Over length |
|---|---|---|
| Starrett 20 Master Precision Square (engineer’s square) | 0.0001 in | Every 6 in |
| Starrett 12 in combination square | 0.0015 in | Over 12 in |
| Veritas Precision Square | 0.001 in | Per inch of length, as Veritas states it |
| Woodpeckers Precision Woodworking Square | No squareness figure stated; scales to ±0.004 in over their length | Repair or replacement if found out of tolerance |
| Crown try square | No figure stated | — |
| Swanson Speed Square | No figure stated; 45° and 90° corners CNC-machined | — |
Figures as each maker publishes them. A scale-accuracy figure (how well the graduations are placed) is not a squareness figure.
How a small error grows
An angular error is a fixed angle, so the gap it opens is proportional to distance. Starrett states 0.0015 in over 12 in for its 12 in combination square; carry the same angle along a 24 in line and the gap is about 0.003 in. That is why a square that seems fine on a narrow rail can show its error across a wide panel — and why the error of a square used to set up a machine repeats in every part cut on it.
Keeping a square worth trusting
A drop can knock a square out of true, and not every square is accurate when new. Check it when you get it, after any fall, and before important layout. The method — draw a line, flip the square, compare — is set out step by step in checking your square; it needs only a board with a straight edge. Starrett describes a more exacting check for precision squares: three squares compared against each other on a flat surface, blades touching, looking for light between them.
Before trusting a square
- It has been checked since it was last dropped or knocked.
- Both the inside and outside of the blade have been checked, if you use both.
- On a combination square, the head and blade are clean, and the lock holds without creeping.
- The stock or head sits flat on the reference, with no burr or dirt under it.
- The reference face or edge itself is flat and straight.
- For a long line, you have allowed for the error growing with length.
Store squares where they cannot fall or be knocked, and protect bare steel from rust; Starrett suggests a light film of petroleum jelly for long storage. Choosing a square to buy is covered in how to choose a woodworking square; gauging lines parallel to an edge, the other half of most layout, is the job of the marking gauge.
Sources
Accuracy figures are the makers’ own statements for particular squares, quoted in the form each maker uses; makers that state no figure are shown as such. Which square suits which job is established bench practice.
- 1.
Frequently Asked Questions on the Starrett Combination Square. L.S. Starrett Company.
Used for: An advertised accuracy of 0.0015 in over 12 in for a 12 in combination square; the protractor accurate to 15 minutes; checking a square against others on a flat surface; storing it with a light film of petroleum jelly.
- 2.
Combination Squares. L.S. Starrett Company.
Used for: What a combination square does — 90° and 45°, depth and height gauge, marking gauge, level and straightedge — and cast-iron or forged, hardened steel heads.
- 3.
20-12 Master Precision Square. L.S. Starrett Company.
Used for: Squareness accuracy to 0.0001 in every 6 in; hardened, ground and lapped beam and blade; not graduated; used when extreme accuracy is required.
- 4.
Veritas Precision Square. Veritas Tools Inc..
Used for: Accuracy stated as that of an engineer’s square, 0.001 in per inch of length; graduated legs; a relieved inside corner.
- 5.
Precision Woodworking Square. Woodpeckers.
Used for: Scales engraved to ±0.004 in total accumulated error over their length, and a promise to repair or replace any square found out of tolerance; no numerical squareness figure on the maker’s page.
- 6.
Crown Tools 126 9 Inch Try Square. Crown Hand Tools.
Used for: A traditional try square: a 9 in hardened, tempered and blued steel blade in a rosewood stock with brass fittings, made in Sheffield; no numerical accuracy stated.
- 7.
Speed Square. Swanson Tool Co..
Used for: A 7 in die-cast aluminium layout square combining try square, mitre square, saw guide, line scriber and protractor, with CNC-machined 45° and 90° corners and a pivot; invented in 1925.
- 8.
16" x 24" Steel Carpenter Square. Swanson Tool Co..
Used for: A 16 × 24 in framing square for rafter and stud layout and squaring large work.
About this guide
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