1. Bombarde 32'. The use for this stop can be found in 19th-20th century French repertoire for Tutti places. Because it is such a low sounding stop, it takes a while for the pipes to be filled with air and to speak properly. Therefore, it is best not to use it in virtuoso pedal parts.
Long notes and passages with double-pedal sound very well with Bombarde (such as in Widor Toccata). Of course, in order to use it, you have to have the reeds at 16', 8' (and 4') in place. If you want to use it in the Baroque music, again, look for slow motion in pedals (such as in Piece d'Orgue, BWV 572 by Bach).
2. Posaune 16'. In addition for loud places in 19th-20th century repertoire, this stop can be very useful in the German Baroque pieces for virtuoso pedal lines and overall pedal sound with the Organo Pleno registration. For the music of Bach, it is generally drawn before Trompette 8'.
3. Trompette 8'. This stop can be used as a chorus reed for loud registrations but it can also be a solo stop where there is a need to bring out the choral tune (cantus firmus). For example, in Plain Jeux registrations of French classical composers the pedal part often takes a choral tune (hint: here you might even want to use reeds 8' and 4' together). In German Baroque chorale preludes and fantasias you can find multiple instances for Trompette. In many cases, this stop works well together with the Octavbass 4'.
Another use of Trompette 8' is when there is a need to play cantus firmus in the tenor with pedals. Matthias Weckmann from 17th-century Hamburg liked this combination: Trompette 8' in the pedal (the tenor part), Trompette 16' alone in the left hand (the bass part). The right hand takes alto and soprano parts with the Principal 8' (and 4' if needed).
4. Clairon 4'. If you want to use this reed for the tenor chorale tune, play one octave lower. For the tune in the alto part (yes, the chorale can be present in any part), play it in normal range. Additionally, Clairon can be very useful as supplement to the full 32', 16', and 8' reed chorus but you have to check the quality of this stop because on some organs it might not be nicely voiced.
5. Cornet 2. The most common use of this reed stop (not to be confused with the manual Cornet which is a compound 5-rank stop) is for playing the soprano cantus firmus with pedals.
You can even play hymns this way: pedal takes the soprano tune with the 2' Cornet, alto and tenor is played on one manual by the right hand at 8' pitch level, and the bass - with the 16' and 8' flutes (or 16' reed) with the left hand. This disposition of voices when the pedals must take the top voice while playing from the hymnal is a perfect exercise for your brain.
When registering your organ pieces with the above reed stops, always take into consideration the acoustics of the space, the balance between the parts, and the style of the instrument and music. On some organs, it is best not to double the reeds with the labial stops at the same pitch level because of clarity and winding issues.
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