Albert Einstein said:Let us take a system of co-ordinates in which the equations of Newtonian mechanics hold good. In order to render our presentation more precise and to distinguish this system of co-ordinates verbally from others which will be introduced hereafter, we call it the “stationary system.”

If a material point is at rest relatively to this system of co-ordinates, its position can be defined relatively thereto by the employment ofrigid standards of measurementand the methods of Euclidean geometry, and can be expressed in Cartesian co-ordinates.

If we wish to describe the motion of a material point, we give the values of its co-ordinates as functions of the time. Now we must bear carefully in mind that a mathematical description of this kind has no physical meaning unless we are quite clear as to what we understand by “time.” We have to take into account that all our judgments in which time plays a part are always judgments of simultaneous events.

Einstein introduces this idea of a rigid measurement, and that his theory is about the kinematics of rigid bodies.

What, though, does "rigid" really mean? In what sense is bouncing light off a mirror and measuring the roundtrip time, a rigid measurement? Light isn't substantially rigid, like a measuring rod made of, say metal, but its velocity is fixed everywhere

**measurements**can be made.