Right. The big problem with accurately bisecting a 180 degree angle is you need to have access to points rather far in both directions from both points. If yo

Right. The big problem with accurately bisecting a 180 degree angle is you need to have access to points rather far in both directions from both points. If you want an accurate right-angle corner near the edge of a property that's flanked on two sides by fences, rivers, busy roads, etc., then you might not have convenient access to one of the anchor points needed to perform your bisection. (Or semi trucks snagging your rope might be inconvenient.) The nice thing about Pythagorean triples for drawing out foundations is that you don't need access to any ground outside the foundation of your building. Being integers, you also don't need any measuring device apart from some rope. You just pace out a bit under 1/3 of the shortest side (or a bit under 1/5 the longest side, whichever is shorter) (call this an "'bout-right") length of rope. You then use your 'bout-right to make a 3'bout-right, a 4'bout-right, and a 5'bout-right piece of rope. Pull the three ropes tight in your perimeter, and you've got your right-angle for your foundation.