For more information on the square, see our beginner site Common Woodworking.

Provided you have a perfectly square square, it’s a simple matter to check a square for square. Most people starting out may not have that luxury. So, you bought a tidy looking square from a carboot sale or flea market or you received one from granddad’s garage tools. How will you know if it’s got the dead square accuracy you must have for your work?

I want my squares dead square with no margin of inaccuracy so I strive for perfect methods to determine their accuracy so I can either correct the erring tool or get rid of it. The process is simple and quick, taking no more than a few minutes. To do this I need two flat boards about 9-10 wide. In this case I am using 12″ wide boards as my square is 12″. Pine is soft and easy and works best of all, but any wood will do.

I edge joint two meeting edges of the boards as I would for any panel making project. I find that a longer plane around 15″ works best for this. My panel wood is 24″ long as I am checking a 12″ square.

The reason for two boards is that one checks the other. Planing the two straight means that the two edges will meet perfectly with no gaps at an part. To do this, start planing in the usual manner from one end to the other, just to eliminate any disparity in levels.  The move to a central point in the length of the boards and begin planing with short strokes working from the centre with longer and longer strokes until you get a continuous stroke from both edges.

Bring the two edges together and if the two meet seamlessly the edges most be dead straight; they cannot be anything but.

You can use either a very sharp pencil or a sharp knife for the next step. Square a line across the board from one side to the other.

Flip the square over and test it against the same line. If the points meet across the width the square the square is square. Also, check along the whole length of the line made with the square after the flipover, any deviation will show that the edge of the square is not straight.

You must check both sides of the blade of the square by sliding the blade along to check the inside edge.

One part remains to be checked. The outer edge of the stock of the square. This is simply a measurement check to see if it’s dead parallel to the inner face. If it is it is also square to the blade.


  1. Paul Sellers on 15 December 2011 at 10:09 am

    Glad you liked it. They are important. The plane’s ones seem to have helped too. My goal is to try to help people think beyond the bench, the tool and the wood. To feel comfortable walking through the woods and smell the earth, feel the rain and the sun’s warmth. The fuller colour of working wood.

  2. Anonymous on 15 December 2011 at 10:55 am


    I am the one who keeps changing the visual theme. Paul is probably as surprised by the constant changes as you are.

    Thanks for the feedback.

    Joseph Sellers

  3. William on 29 August 2012 at 10:49 pm

    Very informative post. This has shed a lot of light on refurbishing my old square.

  4. Julian Louis on 22 April 2016 at 1:19 am

    I’m still having trouble understanding why you need two boards

    • Paul Sellers on 22 April 2016 at 6:57 pm

      Because planing two boards at the same time and bringing the edges together will show and discrepancy as I is impossible to plane one edge round and the other hollow when done at the same time.

  5. Dan on 23 August 2017 at 3:15 am

    get necessity for 2 boards planed in tandem but missed the “rear in vise” vertical edge treatment, maybe in an earlier post? If those verticals were “out”, or tilted just a fraction rear to front in the vise as you were planing, seems they would be doubly so, forming, I guess, a “V” shape when the 2 boards were flipped on side, opposed and meeting tightly at their seam?

    • Dan on 31 August 2017 at 2:45 pm

      sorry for my confusion, verticals “out” a fraction ( any ever so slight “v” shape formed at the ends of the two planed and flipped boards meeting snugly ) would imply the square is not “dead square” ( 90 degrees precisely ) and I missed your post were you write “I have made it a policy never to admit a non-square square to ever remain in the shop so it’s now the square is tested for absolute truth.”
      Understand your “test” and was thinking forward how to fix or adjust the square when I should have dug more deeply to discover your policy

  6. Not all that smart on 27 February 2019 at 2:20 am

    Paul, I respectfully disagree; When a machinist wants to create a straight edge without using a factory straight edge, it takes three pieces of metal to achieve that result. The procedure is much like what you’ve described. The additional steps are to compare edge A with edge B, B with edge C. You keep checking and modifying to the point that any one of the edges matches the other two. This was how the first surface plate was made, how true squares were first made, etc. [I use a machinist’s square, inexpensive machinist’s square are almost certainly within the budget of most home woodworkers and home metalworkers.

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