A later development was the upper quadrant three-position semaphore signal. These worked in the upper quadrant to distinguish them from the then standard two-position lower quadrant semaphores. When the arm is inclined upwards at 45 degrees, the meaning is "caution" and the arm in the vertical position means "clear". Thus, three indications can be conveyed with just one arm and without the need for a distant arm on the same post. The early abandonment of the three-position semaphore signal in the UK allowed for the widespread replacement of two-position lower quadrant signals by two-position upper quadrant signals.Vanns, M.A., (1997), An Illustrated History of Signalling, Ian Allan, , p.75
