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My assumption is that it's so they don't roll away. But if a carpenter spends the majority of his/her day working on a flat surface, a regular pencil wouldn't seem to have the tendency to get away from them.

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    I have a feeling it has something to do with the thickness of the graphite inside. With a flat pencil you can make it thicker in one direction so you can drag it accordingly without it breaking every 2 seconds – Joe Phillips Aug 20 '15 at 14:32
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    I thought it was so that they fit more snugly behind the ear? – Simba Aug 21 '15 at 12:27
  • Henry Petroski's "The Pencil: A History of Design and Circumstance" is likely to give a definitive and exhaustive answer. I haven't read it, so I can't answer, but his other works are excellent. – Schwern Aug 21 '15 at 20:56
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According to wikipedia, it's so they don't roll away::

A carpenter pencil (carpentry pencil, carpenter's pencil) is a pencil that has a body with a rectangular or elliptical cross-section to prevent it from rolling away. Carpenter pencils are easier to grip than standard pencils, because they have a larger surface area. The non-round core allows thick or thin lines to be drawn by rotating the pencil. Thin lines are required for high precision markings and are easy to erase, but thick markings are needed to mark on rough surfaces. The lead is strong to withstand the stress of marking on such surfaces. The pencil is robust to survive in a construction environment, for example when placed in a bag together with heavy tools. The core is often stronger than in other pencils. Carpenter pencils are also used by builders, because they are suitable for marking on rough surfaces, such as concrete or stone. This shape and lead density aid in marking legible lines with a straight edge that are clear and easy to follow with a saw blade.

Though it's possible that there's some historical precedent involved:

The flat pencil is one of the oldest pencil types. The first versions were made by hollowing out sticks of juniper wood. A superior technique was discovered: two wooden halves were carved with a groove running down them, a plumbago stick placed in one of the grooves, and the two halves then glued together—essentially the same method in use to this day.

The "flat so it doesn't roll" theory is also supported by this article on Carpenters Pencils at PencilPages.com

A carpenter's pencil has at least two essential characteristics which distinguish it from other types of specialty pencils. First, the shape of a carpenter's pencil prevents it from rolling, even when placed on a steep sloping roof. The most popular shape seems to be a flat, octagonal style. Secondly, the lead must be strong enough to make a heavy mark and to withstand the stresses of carpentry, which frequently requires marking on very coarse surfaces. The leads are usually wide and flat.

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    "the shape of a carpenter's pencil prevents it from rolling, even when placed on a steep sloping roof" Good to know that it slides off without rolling! – David Richerby Aug 20 '15 at 15:24
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    Also, their shape helps them to be sharpened with a knife, x-acto, axe (if you are brave and stupid) or even sandpaper. All those are readily-available tools for anyone in the business. Also, the care to sharpen them is a lot lower than round pencils. – Ismael Miguel Aug 20 '15 at 18:02
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    You can sharpen one side quickly and the pencil is ready to use again. With a round pencil, you have to have an extra care to sharpen every side equally or you end up writting with wood. – Ismael Miguel Aug 20 '15 at 18:08
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    @DavidRicherby The key is to resist the temptation to chase the pencil as it is sliding off the roof... – Craig Aug 22 '15 at 0:02
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    Probably easier to hold while wearing gloves too. – user14895 Aug 22 '15 at 4:02
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Not rolling is only one aspect.

Line thickness

The shape of the pencil also allows the lead to be rectangular. Which allows you to easily draw lines of various thicknesses, simply by rotating the pencil.

Stronger core

The shape allows for a larger volume of "lead", which increases the strength and reduces breaking.

Measuring and spacing

If you know the dimensions of your pencil, you can use it to make quick inaccurate measurements. It can also be used as a spacer, or standoff. The rectangular shape allows for two different dimensions, as opposed to a single one with a round pencil.

Scribing and marking

Scribing and marking can be a bit easier with a rectangular pencil. The rectangular shape also allows you to easily scribe parallel lines, at two different offsets.

I'm sure there's other uses I've missed, as I'm not a professional Carpenter myself.

  • Not sure if any of these really sound reasonable to me. A round lead would be stronger than a flattened lead; so a as-thick round pencil would be stronger still. Leads will be sharpened to a point usually anyway - unless you're using a pocketknife I guess. Pencils won't be all that accurate measuring devices, given the wood can expand... – Joe Aug 20 '15 at 17:33
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    @Joe If you read the entire quote from the Wikipedia article posted in the other answer, you'll find that most of my points are also mentioned there. When you sharpen a carpenter's pencil, you sharpen it to an edge (usually using a knife) not a point. My pencil is 9/16" x 5/16", and has been every time I've checked it. – Tester101 Aug 20 '15 at 17:35
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    @Joe - I think you are wrong on almost every point. The carpenter pencils I have used over the years have always met every point made by Tester101 and then some. The scribing techniques described are a very good point and really not feasible with a round pencil. – Michael Karas Aug 21 '15 at 11:48
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As a professional carpenter I have used both round and flattened pencils. The most obvious convenience of the flattened pencil is the additional strength created by the flattened piece of graphite. These pencils are usually sharpened to a thin line instead of a point. Since marks are made by moving the pencil lengthwise in the direction of the mark you have at least an eighth inch cross-section of graphite to rub against the rough surface of the wood as compared to the tiny point on a traditional pencil. The flattened surface is also very easy and fast to sharpen with a utility knife which is a tool a carpenter is usually carrying around.

  • Hello, and welcome to Home Improvement. Good answer: keep 'em coming! – Daniel Griscom Feb 17 at 17:34
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WHen laid flat it can draw a line 1/8 inch parallel to a surface. When laid on its edge in can draw a line 1/4 inch parallel to a surface.

  • Never thought about that! I always thought it was the rolling issue. Meanwhile: Welcome to Home Improvement. Please take the tour at diy.stackexchange.com/Tour to get the most out of this site – SDsolar Jun 16 '17 at 0:43
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You can also jam a square pencil into The claw of your hammer to scribe hard to reach places I.e if some one was sheeting a curved balcony and wanted to to scribe the shape of the curve on to the plywood from underneath no time needs to be wasted on moving a ladder.

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