So far, each inertial frame has its own meter bars at rest to measure distances, clocks at rest to measure durations, and elastic solid objects (threads, springs, etc.) to measure forces.

**(Assumption)** Every rigid or elastic solid object can be brought (to be at rest) into any inertial frame.

Backed by the principle of relativity, one can venture to say that:

**(Principle)** A rigid or elastic solid object has the same mechanical (incl. geometrical) properties in every inertial frame.

From this, it naturally follows that:

**(Corollary)** The units of length, time, and force can be synchronized between any two inertial frames K and K'.

To synchronize e.g. length, take a rod that is 1 meter long in K, bring it "up to speed" into K', and adjust the definition of 1 meter in K' accordingly. Similar applies to time and force. After that, the measuring tools of K and K' will be interchangeable in any mechanical experiment.

**(Definition)** Two inertial frames are of the same construction if and only if their units of measurement (length, time, and force) are synchronized.

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