This proposition states that the sum of two angles in a triangle is less than two right angles. In Euclidean geometry, Euclid later proves that all three angles sum to exactly two right angles. In hyperbolic geometry, the three angles always sum to less than 180°. In both cases, this proposition is just a looser version of the theorems regarding the sums of all three angles. To prove that the angle A plus the angle C is less than 180°, an exterior angle is constructed on C, from which the proposition is proved algebraically. Calling the red angle α and the blue one β, the yellow angle is 180°-β. The previous proposition tells us α<180°-β, from which α+β<180°.