Typically small, hand-held devices, clappers have two pieces of wood or plastic hinged together so that they can be brought together to make a sharp "clap". This noise is used as an absolute auditory reference for the synchronisation of the soundtrack of the scene. On most occasions during the filming of the BBC Wales version of Doctor Who, the board has had the traditional colours of "absolute" white and black, arranged in diagonal slants across the clapping mechanism. These colours are used because they're visible in almost any lighting conditions. However, the production team occasionally use multi-coloured clappers.