The art of “voicing” an organ pipe, that is, treating it in order to produce a satisfactory musical tone, is highly developed, but on an entirely empirical foundation. Flue pipes only are dealt with here, chiefly the open diapason, an open metal cylindrical pipe. The several tone‐producing mechanisms in it are described, and some two dozen different variables, each of which can affect the tone, are described with reference to the stage in manufacture in which they occur. Some account is given of the interrelationship of the various adjustments. Finally mention is made of the place of any one pipe in the tonal structure of a complete organ.