separated by a time interval as recorded by a clock at A, then they reach B separated by the same time interval as recorded by a clock at B.
* Reichenbach's round-trip condition: If a light beam is sent over the triangle ABC, starting from A and reflected by mirrors at B and C, then its arrival time back to A is independent of the direction followed (ABCA or ACBA).

Once clocks are synchronised one can measure the one-way speed of light. However, the previous conditions that guarantee