Abstract: Position-sensing apparatus is provided which includes a passive coded surface formed by a pattern element with an arrangement of indicia that together form a two-dimensional windowing pattern; by "windowing pattern" is meant a pattern having the property that any local portion of the pattern of given size in terms of pattern features (a "sub-pattern"), when considered in isolation, is so characterized by its features as to permit the location of the sub-pattern to be determined, for at least one orientation of the sub-pattern relative to said pattern. The apparatus further includes an indicator element movable across the pattern, and a sub-pattern detector with a sensor for sensing the pattern indicia in the locality of the indicator element. The detector is operative to process the output of the sensor to derive sub-pattern data representative of a sub-pattern local to the indicator element in an orientation enabling its position to be uniquely determined.