This paper describes a strategy for on-line interpretation of sketched engineering drawings. It represents the design for The Designer's Apprentice - a pen-based system for producing detailed mechanical engineering drawings on a realistic electronic drawing board. The paper examines the problems of interpreting scanned drawings and assesses how such problems affect on-line systems. It is therefore relevant to on-line and off-line systems. The strategy is enhanced by making an early distinction between annotation and the object outline. This discrimination shapes the subsequent processing: the object outline is subjected to node connecting, face-finding and beautification whereas annotation is classified according to the BS308 engineering standard.