From the second postulate of special relativity, together with homogeneity of spacetime and isotropy of space, it follows that the spacetime interval between two arbitrary events called 1 and 2 is: c 2 ( t 1 − − t 2 ) 2 − − ( x 1 − − x 2 ) 2 − − ( y 1 − − y 2 ) 2 − − ( z 1 − − z 2 ) 2 . This quantity is not consistently named in the literature. The interval is sometimes referred to as the square root of the interval as defined here.