Supplementary angles. Not a linear pair

This object is a carpenter's square, a very useful tool. It shows a pair of supplementary angles that are not a linear pair, because of its perfectly right properties. The carpemter's level is used to measure if a corner forms a right angle, and so, there is more than one right angle inside it. This picture was obtained from the URL http://www.daviddarling.info/encyclopedia/C/AE_carpenters_square.html, using google images.