Square tube and formed steel have rounded corners, especially when you work with larger sizes. the surface of the adjustable square is only about 3/4 of an inch wide,with the blade in the center of that leaving a surface of 3/8 to wrap the corner.  The corners  have a larger radius than that on most sizes!
Maybe one of the Educators can explain it in more detail!

Hello Steven, I will see about posting some pictures of these tools in use. As ctacker explained, with larger tubes the radius on the corners makes using a standard machinist square or tri-square, as some call it, difficult, it can lead to inaccuracies when laying out lines such as would be required to torch cut the tubes and maintain a square cut end. When the square head has an extended surface such as the one in his pictures, it allows that part of the square to set flat against the edge that you are squaring off of. ctacker uses a plate attached to his machinist square to accomplish this, there are tools called speedsquares that have a similar ability, there are blocks that can be attached to framing squares to allow them for use on really large items, some folks will lay pieces of long legged angle iron over the top of the tubes and use the two edges to draw square lines with, some people will bend a piece of fairly wide banding or strapping material and use this for this purpose, or as I stated in another thread, you can take a piece of sheetmetal and break an edge at ninety and use it. I'll still try to come up with some pictures so that these things make a bit more sense. Best regards, Allan

Hello Steven, I had a little time to take some pictures of some examples that might help you to understand the reason for the x-tra plate on the square that ctacker included pictures of. As he said, the large radius on many structural tube shapes makes using the edge a bit difficult to square off of. If you take note of the picture that shows the standard machinist square you'll see an example of this. I didn't include a speed square because I didn't have one here when I put this together, but if you go to most hardware stores and ask for it by that name you'll be able to see how they work, they can be used for both drawing square lines and drawing lines at angles and they also come in smaller and larger sizes depending on your uses. There is a picture included here with the stops on the carpenters square that is set to allow an angle to be drawn on the tube, this requires a bit of right angle trigonometry to initially set up, but it allows you to repeat a particular angled line if you were possibly laying out stair stringers on tube steel, there are similar items that carpenters use called stair gage sets, they also use them for laying out stair treads on wood, but they don't have sufficient depth to allow for the large radius on many square or rectangular steel shapes. There are some other things that you can use too, in a pinch, a wide heavy piece of banding material can be bent at a 90 degree angle and used similarly to the piece of angle that I have included a picture of. Some of these are things that I have used for quite some time and others were brought to my attention by other members of the forum. Hope this is of aid to some out there. Best regards, Allan