Measure both ends at once.

Proper length is measured in the object’s rest frame. An observer seeing the object move measures a shorter length parallel to the motion, provided both endpoints are measured simultaneously in that observer’s frame. Perpendicular dimensions are unchanged.

Events simultaneous in one inertial frame need not be simultaneous in another. This resolves apparent contradictions such as a moving ladder fitting into a shorter barn: the timing of the two door events changes between frames. The travelling twin’s turnaround changes inertial frames, so the twins’ histories are not symmetric.