The parallel cursor and disc. In the Royal Navy manœuvring problems are normally worked out on the Battenburg Course Indicator which gives a mechanical solution of the speed/distance triangle (Fig. 1). A similar device for direct use on the PPI face can be made by engraving parallel lines on the rotatable PPI mask and attaching two perspex vector arms to the centre. Although developed primarily for manœuvring purposes this idea can be put to several purely navigational uses and from it have sprung two valuable adjuncts to the PPI: the parallel cursor and the fixed plotting surface (or disc).