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What is a Carpenter's Square?

Malcolm Tatum
Updated May 17, 2024
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One of the most easily recognized of all carpentry tools, the carpenter’s square is an L-shaped set square that is used for plotting right angles during the construction process. Sometimes referred to as a builder’s square, the carpenter’s square is a flat square that is often made with steel or aluminum, and is relatively lightweight. Carpenter’s squares are often employed in the construction of walls, in the installation of roofing, and with a number of furniture construction projects.

The carpenter’s square of today usually measures twenty-four inches by sixteen inches, which is the largest dimensions ever associated with the tool. Typical rafter square and roofing square models in the 19th century tended to measure eighteen inches by twelve inches. Smaller squares are also available, and may be ideal for smaller jobs.

There are several different ways to effectively use the carpenter’s square. The main function of the device is to lay out and mark patterns that are related to framing jobs. This would include such functions as plotting angles for roofing jobs, the construction of stair steps, and making sure that walls are joined at perfect right angles.

Along with plotting layouts of these types, it is also possible to use a carpenter's square to determine the flatness of a plank or sheet of building material. This is actually an excellent way to quickly qualify materials before purchase or use in a building project. Since many projects require flat building materials in order to be constructed properly, this simple method of checking will prevent the builder from utilizing materials that are slightly warped.

There are essentially three sections of the typical carpenter’s square. The longer arm of the square is referred to as the blade. This section is slightly wider than the second shorter arm of the carpenter’s square, which is called the tongue. The outside corner where the tongue and blade join is known as the heel.

In order to help make calculations quick and easy, the surface or face of the carpenter’s square is equipped with measurements. While in times past, the measuring medium employed was strictly linear, many carpenter’s square models today will also include metric dimensions as well. A carpenter’s square can be purchased at any hardware store, as well as any building supplies retail center.

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Malcolm Tatum
By Malcolm Tatum , Writer
Malcolm Tatum, a former teleconferencing industry professional, followed his passion for trivia, research, and writing to become a full-time freelance writer. He has contributed articles to a variety of print and online publications, including AboutMechanics, and his work has also been featured in poetry collections, devotional anthologies, and newspapers. When not writing, Malcolm enjoys collecting vinyl records, following minor league baseball, and cycling.

Discussion Comments

By behaviourism — On Jan 05, 2011

There are some benefits of using a carpenter's square to measure if something is flat or straight, rather than something like a plumb line. For one, carpenter's squares are less expensive; they're also a lot more durable. I also prefer them because they have multiple uses, and I try to avoid things with only one purpose, since they take up a lot of space without much usefulness.

By Catapult — On Jan 04, 2011

I was taught to use a carpenter's square when I studied theater set building in college, and it really is a necessary tool for pretty much anyone who wants to know the way around a workshop. We used it for measuring angles, for testing flatness, and often as a mold for cutting pieces or drawing and painting scenery flats as well.

Malcolm Tatum

Malcolm Tatum


Malcolm Tatum, a former teleconferencing industry professional, followed his passion for trivia, research, and writing...
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