Irwin lists this stop with the following description: A Foundation stop of 16', 8', or 4' on the manuals, and 16' or 8' on the pedals. It has the essential tone of the Diapason: a strong fundamental and a harmonic series developed to just below the point where dissonant harmonics (not spurious harmonics) enter the tone. Many examples of this stop are moderately soft, and mixture ranks are frequently formed by it. Its unique feature is that the otherwise cylindrical pipes are made just slightly conical in shape for their whole speaking lengths. This form prevents many of the higher-pitched harmonics from being created, and places the energy of the air-column more in the lower chord of harmonics. Those eliminated by this shape are betwen approximately the sixteenth and twenty-third, varying, of course, with scale and wind pressure. The second and fifth harmonics are almost always noticeably strengthened by a conical flue pipe. Nor is the taper so great that the even-numbered harmonics are eliminated, but the odd-numbered are somewhat strengthened, presenting a clear, full tone. The stronger odd-numbered harmonics develop a luminous, clean, but not �cold� tone that is of easy blend with all types of stops. To say the tone is �reedy� or �stringy� does not describe this timbre, these two effects demanding a great number of high-pitched patials. In the treble notes these pipes, even at large scale, have a bell-like clarity without that fringe of shrillness sometimes heard from cylindrical ranks. The bass pipes are strong in fundamental gorund tone and perhaps seem a little duller to the ear, but they contribute powerful components to the ensemble that are very valuable. The conical Foundation ranks are not superior to the cylindrical pipes in the Diapason Chorus, because their sparsity of harmonics will not mesh so well with ranks higher in pitch, such as an 8' with a 4', or a 4' with a 2-2/3' rank. Thus the façade of tone cannot be erected altogether by this type of Diapason. Sometimes just the mutation pitches are conical, being perhaps three-quarters to seven-eighths of the mouth0line diameter at the pipes' tops. Even the tuning collars are conical in form too. In his entry for Cone Gamba, Wedgwood says �The stop known as Cone Diapason is practically a distinction without a difference. Its tone, if anything, inclines rather more to that of a reedy Diapason.� Elsewhere he mentions Pyramid-Diapason as a synonym. Maclean lists Pyramid Diapason as �the name given by Henry Erben to a kind of Gemshorn Diapason or Spitzprincipal�, and cites the St. Patrick's example (see below).
Osiris contains over 50 examples of Spitzprinzipal and Spitz Principal; about half of which are at 4', somewhat less than half are at 8', four are at 2', and one at 16'. It contains two examples of Diapason Conique No examples of the names Cone Diapason, Gemshorn Diapason, Pyramid-Diapason, or Spitz Diapason are known. Contributions welcome.