Are squares really square? #1: What too look for in your next square

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Blog entry by generick posted 12-28-2015 02:37 AM 1519 reads 0 times favorited 6 comments Add to Favorites Watch
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Are squares really square?

Have you ever put two squares back to back and discovered that their backs are not parallel? Shining a light between them reveals gaps and off angle inaccuracies? If they are both at 90 degrees, shouldn’t they line up evenly with very little or no light passing between them? That’s what I always thought…

I was reading an article from Fine Woodworking (issue #59) that describes how several manufactures make their squares (Bridge City, Starrett, and Stanley). As it turns out, squares are only guaranteed to be square on the inside “elbow” of the square, not on the outside.

I don’t know about other woodworkers, but I use the outside of the square to set the fences on all my equipment as well as the blades on my band and table saws. I always assumed that both sides were square.
If you want a square that is square on both sides, you have to buy one that is explicitly built and graded that way.

Am I the only one who didn’t know this?

-- Rick -- I never let my lack of talent get in the way of my creativity.

6 comments so far

View a1Jim's profile


117909 posts in 4184 days

#1 posted 12-28-2015 03:46 AM

All you have to do is test them for square ,Just put the leg on a straight edge of a board near the middle ,draw a line(inside edge or outside) flip the leg over close to the line and see if it lines up perfectly with the line you drew.
I’ve been in box stores an out of 10 framing squares only one passed this test.


View Reaperwoodworks's profile


94 posts in 1542 days

#2 posted 12-28-2015 03:48 AM

Why does this seem like a rip off to all of man kind who has purchased a square?! Have you found a place that labels squares as square on both edges/sides? I’m curious what they call them.

-- Website:, Youtube:

View 000's profile


2859 posts in 1506 days

#3 posted 12-28-2015 03:55 AM

I didn’t know that. However I do check them both on the inside and outside as I’m using them.

View generick's profile


53 posts in 2809 days

#4 posted 12-28-2015 06:16 AM

I’ve found several companies that sell squares that they claim are square on both sides. They are sometimes called precision squares. This list isn’t exhaustive by any means, but here are some links to squares that are supposed to be square on both sides. The only way I would be able to check them is with some type of reference. I suspect that any good machine shop will have a known reference and can check a square.

I have 3 different squares from 2 to 8 inches. None of them line up perfectly on the back side.

I’ve done the line drawing test and it finds those that are significantly out of square rather quickly. After that it becomes more of test of my eyesight than anything else.

Bridge City Tools Works:




-- Rick -- I never let my lack of talent get in the way of my creativity.

View WoodNSawdust's profile


1417 posts in 1784 days

#5 posted 12-28-2015 08:48 AM

I have found that many of the squares sold are not square. I did not realize that squares are only guaranteed to be square on the inside. This seem counter intuitive since most uses (at least for woodworking) use the outside of the square.

I used to ask a friend to take squares to work (at Boeing) to test. They have some device that will accuracy measure squareness. I gave up after several since none came back within a 0.001 inch over the entire length.

-- "I love it when a plan comes together" John "Hannibal" Smith

View BigYin's profile


421 posts in 3023 days

#6 posted 12-28-2015 10:12 AM

adjusting a framing square

-- ... Never Apologise For Being Right ...

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