Hi everybody; I am a bit stuck with negative angles. What I know is that: If we measure an angle from x-axis clockwise direction, the angle is negative. However, I am a bit dissapointed because while I researched the web I see the negative agles only in topics subject to trigonometry. So I need to integrate my angle measurement knowledge from trigonometry to geometry. As far as I remember we never encountered negative angles in geometry lessons while I am at high school. For example we always assumed angles of a triangle are positive. Is there a rule about this? For example, let's say ABC is a triangle. Then is it right or not: ∠ABC = ∠CBA There is a sentence: "The sum of the angles in a triangle is 180 degrees." Can it be deficit? "The sum of the magnitudes of the angles in a triangle is 180 degrees." It sounds better to me. Is negative angles only defined on Cartesian coordinates? If yes, then if I locate a triangle on a Cartesian plane, can an angle have a negative value? I know it is a small detail. But I really need a clarification. If you prefer to offer a geometry book for this kind of definitions I will be pleased to read.