What is a Combination Square?

A combination square is a two-piece layout tool made up of a steel rule (blade) and a sliding head that locks anywhere along the rule. The head has two reference faces: one at 90° for square cuts and checks, and one at 45° for miters. Many models include a small bubble level for quick leveling and a scribing pin (often hidden in the head) for marking. Because the head can slide and lock, you can set repeatable measurements, check depths, and transfer marks with high accuracy.

Key Parts and How It Works

Rule/Blade: Usually stainless or hardened steel with etched graduations. Common lengths are 6 in (150 mm) and 12 in (300 mm). Some rules have both imperial and metric scales.
Square/Miter Head: Cast zinc, aluminum, or cast iron. One face gives a true 90°, the other a true 45°. The inside shoulder rides the rule and includes a spring-loaded lock bolt.
Locking Mechanism: A knurled knob tightens a wedge or spring to clamp the head to the rule. Better locks hold firmly without drifting.
Level Vial: Useful for rough leveling small parts or brackets.
Scribe/Awl: A removable pin stored in the head for marking lines on wood, plastic, or soft metals.

To use it, place the wide face of the head against your workpiece edge and draw a line along the rule. For 90° lines, use the square face; for miters, use the 45° face. Slide the head to set a measurement, then lock it to repeat the same dimension anywhere.

Common DIY Uses

Marking 90° and 45° lines for cuts on boards, trim, and sheet goods.
Checking squareness of edges, end grain, cabinet parts, and tool fences.
Setting tool heights and depths: Adjust circular saw blade depth, table saw blade height, or router bit depth by setting the rule and using it as a depth gauge.
Transferring measurements from one part to another without relying on reading the scale each time.
Locating centers: With a center-finding head (see variations), quickly find the center of round stock or dowels.
Scribing parallel lines from an edge by setting the head to a distance and running it along the edge.
Checking blade or fence angles at 90° or 45° on miter saws and table saws.

Types and Variations

Standard head only: The everyday version with 90°/45°, level, and scribe.
Combination square sets: Include extra heads:
- Protractor head: Adjustable 0–180° for marking and checking uncommon angles.
- Center-finding head: V-shaped head that finds the center of round or square stock.
Rule lengths: 6 in (handy in tool belts), 12 in (most common), 16 in or longer for framing and cabinetry.
Graduation styles: Etched, stamped, painted fill, or black-chrome contrast for high visibility. Some are metric-only or dual-scale.
Materials and grade: DIY-grade (cast zinc/aluminum heads) vs. machinist-grade (cast iron/steel heads, tighter tolerances). Stainless rules resist rust.
Digital angle versions: Some protractor heads include digital readouts, helpful for precise angle setting.

How to Choose a Good One

Accuracy: Test in the store if possible. Place the square face against a straight edge, draw a line, flip the square over, and draw again. Lines should overlap; any V-shape shows error.
Graduations: Look for etched or laser-marked lines rather than painted-only. Dual-scale (imperial/metric) is convenient.
Rule finish: Matte or satin reduces glare and improves readability.
Head material: Cast iron or steel heads stay flat longer; zinc or aluminum are lighter and fine for general DIY.
Locking action: The head should slide smoothly and lock solidly without creeping. Check for play by trying to wiggle the head when locked.
Length: 12 in covers most household jobs; a 6 in square is great for tight spaces and on-the-go work.
Replaceable rule: Some brands allow swapping rules for different lengths or scales.

How to Use It Accurately

Mark a square line: Place the wide face of the head flush to a jointed edge, hold firm with your non-dominant hand, and draw along the rule with a sharp pencil or knife.
Mark a 45° miter: Rotate to the 45° face and repeat the process for trim, picture frames, or corner blocks.
Set a consistent offset: Slide the head to the desired measurement, lock it, then run the head along the edge while scribing a parallel line.
Depth/height gauge: Set the rule to the required depth, lock the head, and use the rule as a gauge to set saw or router depth.
Transfer measurements: Lock the head to a test piece or opening, then bring it to the workpiece and mark—no need to read the numbers if repeatability matters.
Quick level checks: Use the vial to level short brackets or small shelves. For long runs, use a full-size level.

Quick Accuracy Check at Home

Use a factory edge or a jointed board.
Draw a line with the 90° face.
Flip the square left-to-right and place it on the same starting point.
Draw a second line. If the lines overlap, you’re square; if they diverge, the head or rule is off.

Care and Maintenance

Keep it clean: Wipe dust and pitch from the rule and head. Fine grit causes wear and inaccuracy.
Light oil: A drop on the rule keeps sliding smooth and prevents rust. Wipe off excess.
Protect the rule edge: Store it so the edges don’t get dinged. Nicks affect accuracy and pencil lines.
Avoid drops: The head faces need to stay flat. Dropping onto concrete can warp or chip them.
Check periodically: Use the flip test to confirm it’s still true. Replace a bent rule; most heads are not user-adjustable for squareness.

Mistakes to Avoid

Relying on a damaged edge: A cupped or rough board edge will throw off your mark. Joint or choose a straight reference edge first.
Misreading scales: Many rules have multiple scales. Confirm you’re on the correct row (e.g., 1/16 vs 1/32 or metric).
Loose lock: If the head creeps while marking, your lines will be off. Tighten the lock and recheck.
Using the wrong face: Be sure you’re on the 90° or 45° face as intended.
Pushing the scribe too hard: Light, controlled pressure preserves accuracy and avoids slipping.

Related Tools

Try square: Fixed 90° square, no sliding head—simple and reliable.
Speed square: Triangular layout tool for quick 90°/45° marks on framing and roofing.
Framing square (carpenter’s square): Large L-shaped square for layout on framing and stair stringers.
Bevel gauge (sliding T-bevel): Transfers any angle but doesn’t measure it.
Machinist square: Precision fixed square for high-accuracy checks.
Protractor: Measures angles; some combination square sets include a protractor head.

Practical Examples at Home

Hanging shelves: Use the 90° face to mark bracket holes perpendicular to the shelf edge, and the vial for a quick level check before final tightening.
Installing baseboards: Mark 45° miters on trim ends for inside and outside corners, and confirm the miter saw is set true.
Building a cabinet: Check panels for square and scribe consistent reveal lines around doors and drawers by setting a fixed offset.
Setting saw blade height: Lock the rule to the desired height (e.g., 3/4 in), then raise the table saw blade until it just kisses the rule.
Drilling centered holes in dowels: With a center-finding head, mark two lines; their intersection is center.

With a bit of practice, a combination square speeds up layout, improves accuracy, and helps you produce cleaner, better-fitting projects throughout your home.