So, you need to be very careful which scale you use when measuring the angles. With a protractor, the angles of any shape can found where two lines meet and form a corner.

A protractor is used to draw and measure angles. An angle is the amount of turn between two lines. Angles are measured in degrees with the symbol °. Angles can be labelled with 3 letters (eg DEF ...

Use This Simple Trick to Measure an Angle Using Only a Level It's all about the plumb.