Non-Euclidean triangle

According to Albert Einstein, the three-dimensional fabric of space can be curved. In a flat Universe (or on a flat surface), Euclidean geometry applies: the inner angles of a rectangle add up to 360 degrees and those of a triangle add up to 180 degrees.

In non-Euclidean geometry this is no longer the case — for example, on the surface of the Earth (on a large scale). If a triangle is drawn from the pole to the equator, along the equator and back to the pole, its internal angles add up to 270 degrees, not 180.