Vertical and horizontal

In astronomy, geography and related sciences, a line or plane passing by a given point is said to be vertical if it contains the local gravity direction at that point. Conversely, a line or plane is said to be horizontal (or leveled) if it is perpendicular to the vertical at a given point. By extension, the concept applies to finite objects contained by a line or a plane, such as line segments, plane regions, vectors, directions, etc. A surface is horizontal if its tangent planes are everywhere perpendicular to the gravity vector at the tangent point or, equivalently, if the surface normal vector is everywhere parallel to gravity, as in an equigeopotential surface. More generally, something that is vertical can be drawn from "up" to "down" (or down to up), such as the y-axis in the Cartesian coordinate system.