Colour light signals first appeared in the UK in the 1920s. Except for certain removable signals (such as hand signals), signals are installed either on a permanent basis or provisionally (worksite signals, for example). Permanently installed signals are used separately or in groups and may be either fixed or movable, light-based or mechanical (semaphores). A signal is said to be fixed if it always displays the same aspect; it is said to be movable if it can display at least two different aspects corresponding to the "on" and "off" positions respectively for the signal in question. With some signals (such as semaphores), the "off" indication can only be given by the arm moving from the "on" position.