Adlung lists these names with the following description: Sedetze, Sedex, Sedecima, are indeed all the same, and are derived from Sedecim, "sixteen". Accordingly, if one were to proceed upward [from C] for 16 diatonic notes, one would end up not on c['], but on d['], or a second above c['], which does not sound [well] on the organ . Thus it is clear that this must be a mistake in the figuring, and that Sedecima is written instead of decima quinta [i.e., fifteenth]; for it is supposed to be an octave stop, because it is at 2' and 1'. The size of the Sedecima is in proportion to that of the Principal; with a 4' the Sedecima is 1', with an 8' it is 2', and with a 16' Principal it is calculated at 4'. ... Sedex is the same thing by origin; although Niedt in the second part of his Handleitung, first edition, chap. XII, presents it as a Sesquialter, Mattheson in the second edition nevertheless corrects him by saying expressly that it is a little Superoktave at 1' or 2' pitch. The name "Sedetze" appears in Praetorius when he deals with Oktaves . Yet according to correct calculation it ought always to be called quinta decima, or Quintez for short. Grove describes Sedecima as follows: A term found in Habsburg Europe of the 17th and 18th century to denote a 1-1/3' Sifflote. Throughout its period of popularity, [Sifflöte] could be either 1' or 1-1/3', somebuilders preferring the first, others the second. Much the same was true of the Sedecima, the Sifflöte of Habsburg countries. Williams supports Grove's description. Maclean, however, calls Sedecima �an old name for Fifteenth�.
All known examples are given below. No examples of Sedex are known. Contributions welcome.