Although it is a simplification, the fabric of space-time can be imagined as a plane that can be curved into a sphere, a saddle, or a flat surface. In each case, the curvature of space-time would be positive, negative, or flat, respectively. A triangle drawn in a universe with positive curvature would have internal angles summing more than 180°; a triangle drawn in a negative universe would enclose less than 180°. In a flat universe, the angles add up to exactly 180°.