- An angle consists of two rays that share a common vertex. Angles can be classified as acute, right, obtuse, or straight based on their measure. - Congruent angles have the same measure. The protractor postulate relates the measure of an angle to the difference between the measures of the rays that form the angle. - Additional properties include that the measure of an interior angle is the sum of the measures of the angles on either side of it. Adjacent angles share a vertex and one side.