Note 1 to entry: The proper duration $\Delta {\tau }_{\text{AB}}$ is the shortest of the durations $\Delta {t^{\prime }}_{\text{AB}}$ in all frames S′ and for inertial frames, $\Delta {\tau }_{\text{AB}}=\Delta {t^{\prime }}_{\text{AB}}/\gamma$.

Note 2 to entry: For events A and B separated by an infinitesimally small squared space-time interval, the proper duration $\Delta {\tau }_{\text{AB}}$ is the shortest of the durations $\Delta {t^{\prime }}_{\text{AB}}$ in all frames S′ and $\Delta {\tau }_{\text{AB}}=\Delta {t^{\prime }}_{\text{AB}}/\gamma$.

Note 3 to entry: The proper duration is a Lorentz invariant.

Note 4 to entry: The coherent SI unit of proper duration is second, s.