Theatre has long been a hotbed of superstition. Theatre people believe that every playhouse has a ghost or two. The Palace Theatre in London keeps two balcony seats permanently open – just for their ghosts. The Theatre Royal, Drury Lane is said to be haunted by the famous clown Joseph Grimaldi among others. Indeed it is said to be the most haunted playhouse in the world. From not referring to a certain Shakespeare play by its name to the cheery exhortation of break a leg rather than merely wishing good luck, the ghost light comes with its own set of spooky tales.Theatrical people are extremely keen to avoid upsetting their resident ghosts at all costs - this is the purpose of the ghost light. The belief is that in the dead of night when all is quiet and staff have gone home, the burning ghost light enables ghostly performers to put on a show, which keeps them happy and less likely to sabotage a production or otherwise engage in ghostly mischief. Some stage managers say that when they have forgotten to leave on the ghost light, mischief has indeed occurred! Others believe it is bad luck for a theatre to be left in the dark.Theatrical ghost lights have a rather more practical, down to earth explanation. A ghost light's primary purpose is safety. Even when there is no production showing, there are still many staff members who need to be in the building, like cleaners, maintenance personnel, set designers, and others. After hours, a lamp will usually be placed on or near center stage to help staff avoid mishaps like falling into the orchestra pit or tripping over set pieces. And of course it helps to light the way around the theatre auditorium without necessitating the use of main lights. Ghost lights are not strictly a modern phenomenon, however. In the days before electricity, theatres would be lit by gas lamps. It was thought that leaving a flame burning in a gas lamp would reduce pressure build-up in gas pipes, and so prevent a gas explosion.