The fourth postulate states that all right angles are equal. This is arguably Euclid's simplest postulate, and it feels like it deserves to be a part of the Common Notions, which are collections of algebraic and logical postulates. However, this statement becomes a little bit less obvious when thought about in terms of the way that Euclid defines right angles. Euclid defines a right angle to be the angles resulted from when a straight line stands on another straight line making the adjacent angles equal. Rather than using degrees, Euclid quantifies angles in terms of multiples of right angles, for example he would describe the sum of the angles in a triangle equaling two right angles, rather than 180°. In fact, Euclid doesn't use any number or symbols when talking about plane geometry, and instead describes everything verbally. Euclid also doesn't consider angles to always lie between two straight lines, which allows his definition to generalize to hyperbolic geometry. The angle between two lines would then be defined as the angle formed by their tangent lines.​