Layout Basics
You’ve glued up a large panel for a table top. You’re reasonably sure the two edges are parallel, but you know the ends 
aren’t square. And your framing square is too small to span the width of the table. So how do you lay out a square line to trim to?
I’ve used the “3-4-5″ method which takes advantage of the Pythagorean Theorem. You remember what that is, right? It’s what we learned in geometry or trigonometry class.
The sum of the squares of the sides of a right triangle equals the square of the hypotenuse.
or
a² + b² = c²
So, with the “3-4-5″ method of laying out square lines, you substitute like this:
3² + 4² = 5²
or
9 + 16 = 25
So you can measure 3-feet along one edge of your panel, and 4-feet at a right angles to that (I use my framing square as a guide). The distance between those two points should be exactly 5-feet if your first two lines are at a right angle (90°).
James D. Thompson has written a handy set of articles on Layout Basics. In this article, he shows you how to use a compass to mark out a square reference line. It doesn’t require any math and works every time.