ESDU 87023 gives guidance on fastener devices commonly available to prevent joint loosening under severe vibration in service. The bolted assembly will usually be designed with the bolt(s) pretensioned to a prescribed load (see ESDU 86014), and it is methods of maintaining that pretension that are considered. In particular, the double lock nut is examined and tightening procedures recommended. The influence of tolerances on the preload is considered, with a worked example, and the effect of the vibration cycles on loss of preload illustrated in comparison with the use of a single nut. Fatigue strength is also improved, and the reasons for that are explained and the extent of the improvement shown on a stress-endurance (S-N) curve using experimental data extracted from the literature.