If two angles add up to 180 degrees, then they are supplementary. We conclude that since angles A and B add up to 180 degrees, that they are supplementary. Which law or property does this example illustrate?
Hopefully this picture help you understand. Another thing is that I don't think their is a certain law or property that describes supplementary angles but just the fact that the angles have to add up to 180 degrees and the angles in most cases lie on a straight line.
