A theorem (whose origin can be traced back to von Laue and Hermann Weyl) states that Laue–Weyl's round trip condition holds if and only if the Einstein synchronisation can be applied consistently (i.e. (a) and (b1)–(b3) hold) and the one-way speed of light with respect to the so synchronised clocks is a constant all over the frame. The importance of Laue–Weyl's condition stands on the fact that the time there mentioned can be measured with only one clock; thus this condition does not rely on synchronisation conventions and can be experimentally checked. Indeed, it has been experimentally verified that the Laue–Weyl round-trip condition holds throughout an inertial frame.