Apparatus for providing an absolute indication (that is, an indication which is not lost due to interruption of the supply source) of the position of a movable object, such as a machine tool, by means of a series of binary digital elements extending throughout the range of movement to form a chain-code sequence of dissimilar digital patterns. Each position of the tool is identified in dependence on the particular one of those patterns which is then in registry with a reading head, the identification being in terms of the number of electrical pulses needed to actuate a chain-code generator to produce a similar pattern.