All observers agree upon relative speed. Distance depends on an observer’s motion. Proper length L₀ is the distance between two points measured by an observer who is at rest relative to both of the points.

Length contraction is the decrease in observed length of an object from its proper length L₀ to length L when its length is observed in a reference frame where it is traveling at speed v.

The proper length is the longest measurement of any length interval. Any observer who is moving relative to the system being observed measures a length shorter than the proper length