The rule you're discussing is only for right triangles. Isosceles triangles can come in all different kinds of angle ratios (though two angles will always be equal). So if you have two lines connecting the four corners of a square or rhombus, you do know that the resultant triangles will be 1:1:sqr(2), because the diagonals will bisect each other perpendicularly. But a rectangle doesn't create right triangles. (If you want to prove it, try drawing a rectangle that's REALLY long and skinny. Now draw the diagonals. Do those look like right angles?

-t