The interlocked portion of a whole piece of music may be written in the direct or the indirect style. It may also be written in a combination of the two: e.g., section (stage) one in indirect, sections two and three in direct, section four in indirect. Other combinations exist; the important consideration seems to be that the combination exhibit some type of symmetry.
Within the body of a music composition, single, or several, intervals in the stated style of interlock may be replaced temporarily by those of the other style. However, the original intervals must have returned to their rightful
locations by the end of the section. The temporary intervals must be closed before they leave, i.e., any side of the visiting Fourth(s) which was open, must become a closed Fourth again.
The number of Fourths one selects to use for each of the four sections in the interlocked portion varies from three to, usually, four, perhaps five. More can be used, but the composer must then consider the reappearance of intervals from those already working, the limited musical space that characterizes indirect interlock, and similar concerns of a musical nature.
Available pitches in Separation seem plentiful. In addition to its closed 'defining' pitches, each Fourth has open ones: thus, BE, when it opens on its low side, becomes CE. If BE opens on its high side, it becomes BD#. When both sides are open at the same time, BE is CEb. Furthermore, the defining pitches of BE are still present, but in new locations: B moves 'up' an octave from its closed position, whereas E moves 'down' an octave.
In this connection it is important to mention that any interval temporarily included in a section should be closed on both sides when the original Fourth returns.

Part 4, the next installment of Separation Music Theory Notes, will include - at the beginning - some typical blueprints of the three or four Fourths used in a piece of Separation music. ( Since use of five Fourths is unusual, I will not include a fifth one. If interested, readers should be able to figure them out themselves.) These blueprints will have examples of open and closed Fourths in the two styles of interlock.

If readers have question, they should feel free to send them to me as a message or in comments.

P.J.Diehl 3/25/06