 Einstein derived special relativity by considering how the two postulates would apply to a series of thought experiments. The fundamental tool of these thought experiments was a light clock, which consists of two mirrors separated by a distance L, with a photon bouncing between them. Now, each round-trip of the photon is a single tick of this clock. Now such a device is of very little practical use, but it's very simple and easy to visualize in our mind as we step through various scenarios.