A technical pen is a specialized instrument used by an engineer, architect, or draftsperson (British: draughtsperson) to make lines of constant width for architectural, engineering, or technical drawings. It has been also generally called "rapidograph", although that particular name has officially remained a proper name for a line of products made by German company Rotring.

Early technical pens consisted of a small pair of calipers, having one flat and one bowed leg holding ink between them. By adjusting the gap between the legs the width of the line drawn by the pen could be adjusted. Such pens, kept at a constant angle to the paper, were used for ruling lines, but not for cursive handwriting, nor for off-hand flourishes. The Graphos technical pen introduced in 1934 miniaturized the caliper principle and made the points easily interchangeable.The Sheaffer company produced an expensive drafting set which included such pens for use on linen prints. These sets were often presented to a draftsperson upon becoming 'time served', which is marking the end of the apprenticeship.