 Time, in the special theory of relativity, appears in two guises—coordinate time, which has held center stage thus far, and proper time. The histories of material objects, always moving with speeds less than that of light, are represented in Minkowski spacetime by timelike world lines (curves in Minkowski spacetime such that the tangent vector at each point is timelike). Timelike world lines can be parameterized by a quantity, proper time, which is measured by ideal clocks following such world lines.