There are three conventional spatial imensions: length (or depth), width, and height, often expressed as 'x', 'y' and 'z'. 'x' and 'y' axes appear on a plane Cartesian graph. In the 3rd dimension, typically a 'z' is used and is found in functions such as a "z-buffer" in computer graphics, for processing "depth" in imagery. The fourth dimension is often identified with time in physics, and as such is used to explain the non-Euclidean space-time used in Einstein's theories of special relativity and general relativity.