occurred earlier than the one at
or
. Hence our condition of simultaneity is not satisfied and we are forced to the conclusion that events which are simultaneous for one rigid body of reference (the embankment) are not simultaneous for another body of reference (the train) which is in motion with regard to the first rigid body of reference. This establishes the relativity of simultaneity.
This is, of course, only an elementary example of a very special case of the regulation of clocks by light-signals. It may be asked how the mid-point
is found: one might simply fix mirrors at
and