Hermann Zapf was a consultant for the development of the Rotring Art Pen and produced this little (8.4″ x 6.1″) manual for its use. He told me he was dissatisfied with the pen, though: the final version of the nib was much too rigid to allow the use of variable pressure to achieve the tapered effect he describes below. The tapering of strokes, or “reverse entasis”, is very evident in his Optima typeface.