The system of three-position signalling was introduced on the Belgian Railways (SNCB) in 1919, together with a new style of semaphore. This system is the immediate predecessor of current-day light signalling on the SNCB. The use of three positions, horizontal, 45 degrees and vertical, requires enhanced precision of the instruments. To achieve this SNCB uses a set of two cables for each signal and special equipment. Additionally, return of the semaphore arm to the danger position does not depend on the counterweight alone. With the introduction of yet another model of semaphore arm in 1931, the motion on the signal post itself was changed to rod-driven and a clever pulley was introduced to convert the cable motion into motion of the rods with an accompanying improvement in accuracy of the semaphore position.
Information about the proper application of three-position signals can be found in Signal Rule books of the Belgian Railways. Many documents (most of them in French, some in Dutch) have been collected on this invaluable website: