[Z-man]

3-4-5 and how it relates to building a square building: (Note: by 'square' I mean that all four corners are at right angles.)

It is just an extension of the pythagorean theorem:
a^2 + b^2 = c^2. (The squares of the two sides of a right angle triangle equals the square of the hypotenuse)
Thus:
3^2 + 4^2 = 5^2, or 9 + 16 = 25.

Now, go to a corner of the building you are constructing. Measure and mark a point out 3 meters or feet along one plans for the wall starting at the corner. Then measure and mark out 4 meters of feet along the other "wall." Now connect the point marks (the hypoenuse) in a straight line. If your corner is 'square' you should be reading 5 meters (or 5 feet).

Bonus: to ensure that your building has right angle corners (square or rectangle building), Measure the two diagonals between opposite corners. They should be equal. (Note: this doesn't assure that your buidling is a square, since it works on rectangle building as well)

-Z