The special theory of relativity concerns relationships between measurements made by pairs of observers who are in relative motion. It’s called the special theory of relativity because it's restricted to a special type of relative motion in which one of the observers moves with constant speed in a fixed direction relative to the other. The special theory shows that when two observers share this so called uniform relative motion, each will find that a clock carried by the other runs slow (an effect called time dilation), and each will find that an object carried by the other will shorten along the direction of the relative motion (an effect called length contraction). According to the theory these phenomena are not the result of any peculiarity in the construction of the clocks or the composition of the objects, rather they are indications that time and space themselves are different for the two observers. The clocks and objects simply serve to reveal those differences.