Combination square

How it works

The blade runs in a central groove in the head. A lock bolt with a nut passes through the head and engages the groove, so the blade can be set at any position and clamped. With the blade clamped, the broad face of the head is held against an edge or surface and the blade stands at a right angle to it, which makes the tool a try square of adjustable length. The second face of the head stands at 45 degrees to the blade.

Because the blade can be set and locked, the same tool serves as a marking gauge, a depth gauge, a height gauge and a rule when the head is removed. Starrett’s 1935 catalog lists these uses together with the miter, the level and the center head, and says the combination saves cluttering the bench with several tools each used only rarely.

Anatomy

Blade (rule): Graduated steel bar with a groove along its length. The two faces carry different graduation patterns, and Starrett says its blades are chemically etched steel hardened to Rockwell C 44–46.
Head (stock): The body that carries the blade. Starrett’s 1879 patent describes a stock with a recess for the bar and a boss that carries the clamp, one working edge at a right angle to the bar and the other at 45 degrees.
Lock bolt and nut: A threaded bolt with a tang that rides in the blade groove, a nut, a spring and a special washer. The spring holds the nut off the head so the blade can be slid and reversed.
Spirit level: A vial set in the head parallel to its 90-degree face and protected by an arch, as claimed in the 1879 patent.
Scriber: A pointed scriber held by friction in a bushing in the head; the 1935 catalog says it projects from the bottom of the stock where it can be taken out quickly.
Center head and protractor head: Alternative heads that clamp on the same blade for finding the center of round work and for measuring angles.

Variants

Cast-iron head: Starrett’s Catalog 11 family; the company says it suits woodworking and measures about Rockwell C 25.
Hardened steel head: Starrett’s Catalog 33 family; the company says it is heat treated and precision ground, between Rockwell C 58 and 60, and is recommended for work that alternates between wood and metal.
Center head: Placed on the blade in place of the square head to find the center of a round part. Starrett’s 1935 catalog lists it as an auxiliary head.
Protractor head: Starrett’s bevel protractor head, made in reversible and non-reversible forms, accurate to 15 minutes of a degree according to the company.
Sizes: Starrett lists heads for 4-inch, 6-inch and 12-to-24-inch blades, blades up to 48 inches for the larger head, and an oversized Catalog 8 head with a 24-inch blade.

Specifications

Advertised squareness of a 12-inch tool · in per 12 in: 0.0015Starrett’s stated accuracy for its 12-inch combination square.
Blade graduations · fractions or decimals of an inch, or mm: Starrett 4R: 8ths, 16ths, 32nds and 64ths. Starrett 16R: 32nds, 64ths, 50ths and 100ths. Metric blades are also made.
Blade length · in: 4, 6, 12, 18, 24Heads are made for 4-inch, 6-inch and 12-to-24-inch blades, with longer blades available for the larger head.
Protractor accuracy · minutes of arc: 15Starrett’s stated figure for its protractor head.

Made to

DIN 875
BS 939

Safety and care

Care

For long storage Starrett says an old machinist’s practice is to coat the exposed surfaces with a light film of petroleum jelly.
Starrett offers blades with a satin chrome plate, which it says makes the tool easier to read and increases protection from rust.

Tips

Test the head against two other squares
Starrett describes a check that does not depend on any straight edge. Stand two squares with their blades vertical and reversed on a flat surface so the blades touch, look for light between them, then repeat with the third square against each of the others. Any error shows as light, and the head is then adjusted.1
Flip the head to check squareness against a board
Hold the head against a board edge and scribe a line along the blade, then turn the square over and scribe a second line. If the two lines are not parallel the head is out of square. Starrett notes the weakness of the method: it assumes the board edge is straight.1
File the seat to bring the head back to square
Starrett says the second arm of the head gives a seat for the blade, and that if the blade is found out of square, slight filing of the raised seat re-establishes square. The company also states that the head is calibrated to square with its own blade and is not meant to be used as a square without the blade.1
Reverse the blade to change the graduations
The two faces of a blade carry different graduations. Unscrewing the lock nut frees the special washer from the cutouts in the frame; setting its points back into the slots turns the lock-bolt tang half a turn, and the blade then slides in with the other face showing. If the blade will not re-enter, the lock bolt is too tight and should be pushed back to compress the spring.1
Use the blade as a marking gauge, depth gauge or height gauge
Starrett’s 1935 catalog describes setting the blade at the required distance, clamping it, and sliding the whole square along an edge as a marking gauge. Set at a distance, the blade measures the depth of a mortise; with its end flush with the head it serves as a height gauge where micrometer accuracy is not needed.1
Find the center of a round part with the center head
Put the center head on the blade, place its arms against the round part with the blade across the end, and draw a line along the blade. Turn the head about 90 degrees and draw a second line; the lines cross at the center. Starrett adds that if several lines do not meet at one point, the part may not be round.1
Mark through the holes in the blade
The 1879 patent describes holes through the bar at half-inch or other regular spacing, sized to pass the point of the scratch pin so gage marks can be made when laying out mortises or tenons, and holes near the end countersunk to take a pencil point.1

History

Earliest documented

6 May 1879 (application filed 26 February 1879)

Athol, Massachusetts, United States

US Patent 215,024, “Improvement in Try-Squares,” issued to Laroy S. Starrett of Athol, describes a graduated steel bar with a groove along one side that slides in a recess in a stock and is held by a hooked clamp with a counterbored nut and helical spring. The stock has one working edge at a right angle to the bar and one at 45 degrees, a spirit level protected by an arch, and a socket for a scratch pin. Starrett disclaims, as a broad claim, a slide bar and stock with edges at 45 degrees to each other, one of them at a right angle to the bar, so the patent records that part of the arrangement was already known. The identity of that earlier instrument is not given.1

The combination square joined three existing tools: the try square, the rule and the marking gauge. Starrett’s 1935 catalog describes it as a tool that does the work of an ordinary try square but with a head that slides along the blade and clamps at any place, and adds that it combines a level and a miter. The catalog presents the saving as the purpose: one tool in place of several, each of which would otherwise be used only rarely.

The 1879 patent shows what Starrett considered new. He did not claim a slide bar in a stock with a 45-degree edge, which he stated was already known. What he claimed was a stock with a recess and boss that cover and protect the clamp, a hooked clamp drawn up by a spring-loaded counterbored nut so that dirt does not work between the bar and the stock and throw the bar out of its right-angled position, and a stock that also carries a spirit level and a scratch-pin sheath. The claims concern keeping the blade clean and at a right angle to the working edge.

The tool then grew by attachment. The company says the center head followed on 29 June 1880 and the protractor head on 24 October 1883, each fitting the same blade. The 1935 catalog describes the center head as converting the tool to find the centers of shafting and other cylindrical pieces, and a second blade clamped to the head as a way to scribe parallel lines. The company’s present FAQ says it offers the tool either as a set or as separate parts.

Materials split the market in two. The company says cast iron is soft enough that thin sheet metal rubbed along it will scratch it, and that cast heads suit the woodworker who takes reasonable care, while hardened steel heads suit a user who works wood one day and metal the next. In the 1935 catalog the argument is made for the blade: the hardness of the blade is said to prevent the corners from wearing round and destroying the graduations. These are the manufacturer’s statements; no independent wear test was located for this entry.

What the combination square displaced, in the company’s own words, was “a whole set of common try squares.” The 1935 catalog calls it a complete substitute for such a set, and says it is also a convenient depth gage and a way to square in a mortise.

Timeline

1877
The L. S. Starrett Company says its records show Laroy S. Starrett began the invention of the improved try square in this year.1
26 February 1879
Starrett files the application for what becomes US Patent 215,024. The company’s FAQ gives this date as the day he acquired the patent.1 2
6 May 1879
US Patent 215,024, Improvement in Try-Squares, is issued to Laroy S. Starrett of Athol, Massachusetts. Its three claims cover the stock with its bar recess and boss and clamp, the hooked clamp with counterbored nut and spring, and the stock with spirit level, guard arch and scratch-pin sheath combined with the slide bar.1
1880
The Starrett company dates its founding to this year. Its FAQ gives 29 June 1880 as the date of the center head, the second of the tool’s attachments.1 2
24 October 1883
The date the company gives for the bevel or protractor head, the last of the original attachments.1
1895
Starrett’s Catalogue and Price List No. 13, dated 1895 by the Internet Archive, lists the “Starrett’s Patent Combination Square” as its No. 11, with a double steel square and a thin steel try square listed on the same pages.1
1902
Starrett Catalogue No. 17, dated 1902 by the Internet Archive, lists the patent combination square in several numbered forms, including a model with a metric blade and “New Combination Squares” in larger sizes, together with a combination set.1
1935
Starrett’s Catalog No. 25 describes the combination square as a rule, square, miter, depth gage, height gage, level and center head in one tool and sells it with a hardened blade, with prices from $2.10 for a 4-inch size with center head.1
2024
Starrett’s published FAQ describes two current families: cast-iron heads (Catalog 11) and hardened steel heads (Catalog 33), both made in Athol, with a stated accuracy of 0.0015 inch in 12 inches.1

Where the sources disagree

Which date is the date of the combination square?

The invention began in 1877, according to company records.1
The L. S. Starrett Company (FAQ)
Starrett acquired the patent on 26 February 1879.1
The L. S. Starrett Company (FAQ)
The application was filed on 26 February 1879 and the patent was issued on 6 May 1879.1
The patent document itself

Gaps in the record (6)

The earlier slide-bar-and-stock instrument that the 1879 patent disclaims is not identified in any source read.
Starrett’s 1880 and 1883 attachment dates come from the company FAQ alone; the corresponding patents have not been located and read.
A reissue of the 1879 patent was reported in a search result but has not been fetched, so it is not cited.
No independent test of squareness or of cast iron against hardened steel was found.
No sourced safety guidance and no sourced standard text for the combination square were located; the standards listed are candidates to be resolved.
The 1895 and 1902 catalog scans are poorly OCR-recognized; the entries cited from them are limited to wording that could be read.