It is thought that originally clocks indicated the passing of time by striking a bell once each hour, a dial probably being added to allow the clock keeper to see when the next hour was to be struck. The earliest form of automatic striking work is the count-wheel mechanism, also termed 'locking plate' or 'count plate'. Count-wheel striking work has the disadvantage of sounding the hours in succession; thus, if the mechanism is tripped accidentally, the hands and striking get out of synchronism. See ( Pic 1 ). The rack and snail striking works was developed towards the end of the 17th century. It has the advantage that the relationship between the hands and striking cannot readily by lost. In quarter striking work, many different layouts are used, but the action may be understood by careful examination. See ( Pic 2 ).