The 3-4-5 rule for making square lines or squaring things up.
You can use this for laying out a soccer field or making sure two pieces of wood are at right angles. 32 + 42 = 52 , so 9 + 16 = 25 get it?
Here's a nice little video showing it. He had you make strings for laying it out, but you can just measure one side in any multiple and the other side in the same multiple. For example instead of 3' you could make it 5X that or 15', and instead of 4' make it 20'.