This is my old framing square. It has taken a lot of abuse, mostly from things I have done in attempts to make it more accurate that did not turn out as well as intended. I have thought about simply replacing it.
I looked at new framing squares on-line. Reviews were mixed. Some found the square they purchased to be square. Others found the same makes and models not to be square.
This Instructable will show an easy way to check a square accurately in a store before buying, and it will offer a surprise to consider when looking for a very accurate square.
Step 1: A Little High School Geometry
"Two lines perpendicular to the same line are parallel to one another." is a first-year geometry theorem. No one was watching me in the store, other than the security cameras, so, I pulled three identical squares from the rack and spread them out on the floor. Their labels did not wrap around the square, which would have compromised my test but the labels were only stuck on the surface of the squares. I used one square as a straightedge. I slid the other two squares against this side. Then I slid them toward each other. I was careful to be certain the squares rested firmly against the straightedge. If the squares are really square, the vertical edges should meet along their length. Notice the gap between the edges where the two squares meet each other, especially how it forms a shallow "V" that becomes wider at the upper part of the photo. These squares are not square. See the second photo from a close-up photo of the gap.
I had checked framing squares this way in two other stores, but had not thought to take a photo. These squares are shorter than a framing square. This particular store had only one framing square on the rack, so I chose to test these shorter squares. The framing squares I checked in other stores all had a "V" gap, too; but, the gap on the squares shown here was the most severe.
One lesson learned is that squares off the rack in all price ranges may be accurate enough for framing houses with 2 x 4s, but those I checked are not good for precise work, like making furniture, without making adjustments with an anvil and a ball peen hammer. (This method for adjusting a square makes a dimple that brings the legs of the square nearer to each other or pushes them apart, depending on where the dimple is placed in the corner.)
Step 2: Surprise!
This is called an "L" Connector. They are in the portion of big box home improvement stores where you find corner brackets and similar things for connecting joists. I had long wondered about them and they are surprisingly accurate as squares. The legs of the larger "L" Connector are 12 inches in length each.
Step 3: Test
I used three "L" Connectors to apply the test I showed in step 1. All of the edges met very nicely. See also the second photo. Whereas there was quite a gap between adjoining edges in the second photo of step 1, here the edges meet very tightly. The apparent gap at the very end of the two "L" Connectors is not due to any lack of squareness, but the edges are made with a little recess on each. The edges are still parallel.
If you choose to purchase an "L" Connector for use as a square, do not take what I have presented here at face value, but pull three from the rack and lay them out on a flat surface as I have shown here and check them, yourself.
Step 4: Applications
I chose to buy an "L" Connector just as a reference check for my other squares. It would suffice for an inexpensive square with very good accuracy. It could easily be adapted for use as a try-square. See this Instructable. If done carefully, the legs of the square could be extended by placing the legs against something straight. Bring a straight bar against the straightedge and clamp it to the "L" Connector. Drill holes and rivet or bolt the pieces together very firmly.
I did make one little modification to the "L" Connector. The inner corner of the "L" Connector came with a round corner. That would make it difficult to use as shown in the photo. I used a file to make the corner square.
I am still thinking about whether I will work at improving my old framing square or replace it. The "L" Connector used as a square has given me a better idea than I had before of the exact problems that need correction.
UPDATE (April 25, 2013): I was at Harbor Freight last evening and tested their framing squares. They were accurate, so I bought one. Today I was also at an Ace Hardware and their framing squares were accurate, too. My old framing square will be used with welding projects that need to be somewhat square. Since it is far less than perfect in several respects, it will not matter if weld spatter encrusts it.