Because some platforms on the London Underground are curved and the rolling stock that uses them are straight, an unsafe gap is created when a train stops at a curved platform. In the absence of a device to fill the gap, some form of visual and auditory warning is needed to advise passengers of the risk of being caught unaware and sustaining injury by stepping into the gap. The phrase "Mind the gap" was chosen for this purpose and can be found painted along the edges of curved platforms as well as heard on recorded announcements played when a train arrives at many Underground stations.