The Designer's Apprentice is a pen-based system for producing detailed mechanical engineering drawings on a realistic electronic drawing board. It is being developed with the intention of combining a natural interface, using pen and speech, with an intelligent interpreter which supports sketching and detailed drawing in design. It will recognise cursive script, hand-printed text, symbols and gestures. This paper discusses the issues relating to the interpretation of engineering drawings and suggests how a strategy for an on-line system may differ from that of an off-line system. The paper is therefore relevant to both on-line and off-line systems. The availability of temporal information gives on-line systems a distinct advantage in several cases. Most importantly the use of speed helps to detect corners and the order in which strokes are drawn aids the grouping of elements to form annotation symbols.