Magic 3

I do not actually recall where anyone says what a magic number 3 is! My background is actually Metal work. One thing used a great deal where hand tools are involved especially Marking out on metal is a surface plate. I see Paul has his own version which is a granite tile that he uses for Plane Sole and Chisel flattening and polishing. The most accurate way to check a surface plate is to have 3 of them. Bear with me I am not suggesting getting 3 or even 1. But to check a surface plate for flatness you use a marking grease. The one I know best is called Micrometer blue. What you do is put a small amount (and I mean small one tiny bit on your finger and you will leave a trail all the way home for miles)you spread the Blue out reasonably evenly. it will be almost transparent if you do not spread it out it will look a solid blue and that means you are using too thick a level of it so it could span a gap a couple of thousandths of an inch. Place the second surface plate on top of the first and move around slightly. Now two plates can be out of flat but but exactly mate. If one plate is concave and one convex it could show as flat where blue is transferred in very large areas across the second plate. So then you introduce the 3rd. but you place on the other 2 in turn
it could mate with a convex if it was concave but then the other plate has to be also concave to mate with the first. Ok what has this to do with woodworking? How do you check your square? if you have 3 squares ( I have squares in different sizes and also for metal working) you do the same
place you square on a flat surface and glass is good (See did not try sell you a surface plate) align the 2 blades touching if they mate wonderful but they do not necessarily need to be 90 degrees for this. introducing a 3rd square you cannot get all 3 to mate in pairs if there there is any “out of square”. So check your own squares don’t trust that some person did not drop or hit it. What else? Winding sticks!! I use 3. To check for flat and level of the winding sticks now you know is easey-peasy and has more than one advantage. A longer piece of timber can twist more than one way. And you need to move 2 sticks around to find these differences. 3 sticks on long timber shows not only twist it actually shows bow as well.because when you sight the sticks normally you are looking at 2 surfaces but try this
push any stick in the ground then push a second a distance away. place a 3rd in between and you can align it accurately by sighting the 2 ends and noting the middle? I hope this gives someone an argument to say 3 is not magic. Now you do not need a metrology lab to calibrate your own flat and right angled tools.