The deviation of a Riemannian manifold "X" from the standard metric on Euclidean space is measured by sectional curvature, which is a real number associated to any real 2-plane in the tangent space of "X" at a point. For example, the sectional curvature of the standard metric on CP"n" (for ) varies between 1/4 and 1. For a Hermitian manifold (for example, a Kähler manifold), the holomorphic sectional curvature means the sectional curvature restricted to complex lines in the tangent space. This behaves more simply, in that CP"n" has holomorphic sectional curvature equal to 1. At the other extreme, the open unit ball in C"n" has a complete Kähler metric with holomorphic sectional curvature equal to −1. (With this metric, the ball is also called complex hyperbolic space.)