Set Out Square, Centre, Angle & Depth With Confidence

Before metal is cut, drilled or welded, somebody has to establish what straight, square & correctly positioned actually are. Starrett squares & combination sets give machinists, fabricators, toolmakers, maintenance workshops & skilled trades a physical reference for checking 90°, setting 45°, measuring depth or height, finding centres & laying out work before an error becomes part of the finished component.

A basic square head handles very different work from a centre head or protractor. Start with Measuring & Layout Tools when you're building a broader marking & setup kit, or move into Metal Cutting & Grinding Tools when accurate layout is the preparation for cutting, drilling or material removal.

Starrett has an unusually direct connection to this tool category: founder Laroy S. Starrett developed the combination square concept in the late 1870s, before The L.S. Starrett Company was established in 1880. The principle remains useful today — one accurate rule becomes considerably more capable when the correct head is fitted to it.

What Are Starrett Squares & Combination Sets?

A Rule Becomes a Layout System When the Head Can Move

A Starrett combination square uses a graduated steel rule with an adjustable head that can be positioned along the blade.

Rather than being limited to one fixed reference, the arrangement can support several workshop operations from the same tool.

Depending on the square or set, the range can include:

► Combination squares with an adjustable square head
► Combination sets bringing multiple heads together with a compatible rule
► Square heads for 90° & 45° reference work
► Centre heads for locating the centre of round or square stock
► Protractor heads for setting & reading angles
► Double squares with adjustable sliding blades
► Solid precision squares for dedicated squareness checking
► Rules & blades with different lengths, finishes & graduation formats
► Replacement heads, blades & accessories for compatible combination-square systems

That modular approach is what separates a combination set from a simple fixed square.

Why Choose Starrett Squares & Combination Sets?

This Tool Category Helped Start the Starrett Story

Long before combination squares became familiar workshop equipment, Laroy S. Starrett was looking for a more practical alternative to the fixed-blade try squares he had used as a patternmaker.

His improved square-head design ultimately became the foundation for the combination square & an important part of the company established in Athol, Massachusetts in 1880.

That history matters here because the combination square isn't a peripheral Starrett product.

It's closely tied to why the Starrett tool business exists at all.

One Blade Can Answer Several Different Setup Questions

Move the square head along the rule & the tool can perform operations that would otherwise require several separate references.

Depending on configuration, that can include:

► Checking 90°
► Establishing 45°
► Measuring depth
► Measuring or transferring height
► Marking parallel lines
► Using the blade as a rule
► Checking level or plumb on suitable square heads
► Finding the centre of round stock with a centre head
► Setting or checking angles with a protractor head

The value isn't simply having several functions.

It's having those functions built around a common rule & reference system.

Layout Accuracy Matters Before the Expensive Work Begins

A hole drilled in the wrong location remains wrong even if the drill itself performs perfectly.

The same applies to a cut made from a poor reference line or an assembly welded from components that weren't checked for square.

For practical workshop work, measuring & layout are often the point where mistakes are cheapest to find.

Why Good Fasteners Matter More Than the Tool You Drive Them With makes a related point from the fastening side: the finished result depends on more than the powered tool doing the final operation.

Common Applications

Machine-Shop Layout

A machinist may need to establish a shoulder location, mark a line parallel to an edge, check a component for square or transfer a dimension before machining begins.

An adjustable square head allows the rule to be positioned around the work rather than restricting the operator to one fixed configuration.

Double squares offer another compact adjustable format & are particularly associated with machinists, toolmakers & patternmakers.

Where the square is part of a wider bench setup, Workshop Equipment connects layout work with the environment where components are held, prepared & checked.

Fabrication Fit-Up

Steel doesn't care whether the drawing was correct if the actual pieces are positioned incorrectly.

Before joining, a square can help verify component relationships & establish layout marks.

A 45° reference can assist suitable mitre work while a protractor head expands the setup beyond the fixed angles available from the standard square head.

Once the geometry has been established, Welding Equipment supports the permanent joining stage.

Finding the Centre of Round Stock

Finding a reliable centre on round material by eye defeats the point of precision layout.

A centre head changes the job.

Mounted on the rule, its faces locate against the circumference of the work. Scribing intersecting lines allows the centre to be established.

For work that isn't perfectly round, using more than two intersecting lines can help reveal the most useful centre position.

That makes centre heads relevant to shafts, round bar, discs & other cylindrical workpieces.

Angle Setup & Transfer

Not everything belongs at 90° or 45°.

A Starrett protractor head allows the rule to be positioned at other angles for layout, setup & checking.

Starrett's combination-set protractor designs use a direct-reading double 180° scale, giving the operator a mechanical method for working above or below the blade.

For jobs that move further into dimensional verification, Testing & Measurement Tools expands the workshop setup beyond layout alone.

Carpentry, Joinery & Installation

The combination square isn't confined to metalworking.

Checking square ends, setting repeated offsets, marking parallel lines, establishing 45° references & transferring depths all have practical value in timber work & installation.

A pencil may suit general timber marking while a finer marking method can be chosen when the work demands a narrower reference line.

Carpentry Tools & Fasteners supports the cutting, assembly & fixing work that follows the initial set-out.

Product Selection Guide

Buy the Head Configuration Around the Work

A combination square can be purchased as a straightforward square-head tool or as part of a more capable combination set.

The extra components only create value when you'll actually use their functions.

Combination Square or Complete Combination Set?

If most of your work involves checking square, marking 45°, measuring depth or establishing parallel layout lines, a combination square with square head may cover the majority of the job.

A combination set becomes more useful when the work regularly includes round stock or variable angles.

Adding a centre head provides a practical way to locate centres.

Adding a protractor head introduces adjustable angular measurement & layout.

Buy the additional capability because the work requires it — not because a larger set looks more complete in the case.

What Does the Square Head Actually Do?

The square head is the everyday working component of the combination square.

Its precision faces provide 90° & 45° references, while the adjustable relationship between head & rule enables depth, height & parallel marking operations.

Selected Starrett square heads also incorporate a spirit level & scriber.

Slide the head.

Lock the required position.

Use the appropriate face or rule edge for the operation.

That adjustability is the reason the combination square can replace several simpler layout tools in suitable work.

When Is a Centre Head Worth Having?

Round work changes the reference problem.

There isn't a straight outer edge to measure from in the same way as rectangular stock.

A centre head straddles the circumference of round material so the rule can pass across its centreline.

Mark one line.

Rotate the tool or workpiece.

Mark another.

The intersection establishes the centre.

Anyone regularly working with shafts, round bar, discs or turned components has a stronger reason to consider this head than someone working almost exclusively with rectangular stock.

What Does the Protractor Head Add?

Angles beyond the square head's fixed references require another approach.

A protractor head rotates relative to the rule, allowing the blade to be positioned at a selected angle.

That expands a combination set into:

► Angular layout
► Angle checking
► Machine setup
► Fabrication preparation
► Transfer of angular relationships
► Work that doesn't conveniently fall at 90° or 45°

Starrett's complete combination sets can bring the square, centre & protractor heads together around one rule.

Combination Square or Double Square?

They overlap, but they're not identical buying decisions.

If you want one tool to cover broad everyday layout, the combination square is the natural starting point.

Where compact adjustable squareness work dominates, a double square deserves consideration.

Expert Tips

Clean the Contact Faces Before Blaming the Square

A burr, chip or piece of workshop debris trapped between the head & workpiece creates its own reference.

Before checking squareness, wipe the contact faces & inspect the workpiece edge.

A precision tool only references the surfaces touching it.

Don't Let the Scribed Line Become the Biggest Error

A thick or poorly controlled line introduces uncertainty before cutting begins.

Use a marking method appropriate to the material & required precision.

On metal, the built-in scriber available with suitable Starrett combination squares can create a finer physical reference than a broad marker line.

Lock the Head Before Transferring a Setting

Once the required offset or depth has been established, make sure the head is secured before moving the tool to another position.

A setting that shifts during transfer defeats the purpose of establishing it accurately in the first place.

Use More Than Two Lines on Questionable Round Stock

Perfectly round material makes centre finding straightforward.

Damaged, irregular or imperfect stock can produce slightly different intersections.

Scribing additional lines from different positions gives you more information about the true geometry of the workpiece.

Check the Reference Edge Before Checking the Part

A square needs a suitable edge or surface to register against.

Burrs, weld spatter, damage or rough-cut edges can prevent the head from seating properly.

Prepare the reference before deciding the component itself is out of square.

Don't Treat the Rule as a Pry Bar

The sliding blade is part of a precision system.

Avoid bending, levering or using it for jobs that can damage the straightness, graduations or edges.

If a blade or head is damaged, Starrett offers compatible replacement components & accessories for suitable combination-square systems.

Store the Set So the Heads Aren't Fighting Other Tools

Combination-set components have machined reference surfaces that deserve more protection than the bottom of a loose toolbox drawer.

Keep heads, rules & accessories clean & protected between jobs.

For practical workshop care guidance, How To Clean Your Gear Without Ruining It covers broader equipment-cleaning habits without treating every tool the same way.

Teach the Reference, Not Just the Tool

Knowing how to slide a square head isn't the same as understanding where the measurement originates.

Apprentices benefit from learning why a clean datum, narrow marking line & stable reference edge matter before they start chasing finer graduations.

The Apprentice's First Tool Kit: What You Actually Need looks at building useful trade capability rather than simply filling a toolbox.