Lead with Your Head
When you play the organ and want to be able to conduct the choir, you can lead with your head. This means that whenever you need to show your choristers the entrances, simply use your head movements. Similarly, your choir members will know from your head when to stop singing. Just imagine that your head is your arm and make small but exact movements. In order to show your choir the entrance, try to move your head downward and at the same instant upward with one crisp motion.
Play Three Voices in the Right Hand
If you play the hymn on the organ and need to conduct your choir, you will need at least one free hand to do that. This means that you should take soprano, alto, and tenor in the right hand (because the hymn tune is in soprano) and play the bass with your feet on the pedals. However, quite often you will find that because of the open chord position, it is not possible to play more than two voices with one hand. If this is the case, another option would be to rearrange the chord position into a close position.
In open position chord, the three upper voices can be more than an interval of the fourth but not more than an octave apart. The simplest means to achieve the close position while maintaining the soprano part intact is by flipping the alto and tenor voices. Imagine a C major chord in an open position, such as c-g-e1-c2. The alto takes the e1 and the tenor – g which requires two hands to play the chord. Now flipping the alto with the tenor you will get c-e1-g1-c2 and you can play the three upper notes with the right hand easily. The entire hymn can be rearranged this way while preserving the original voice leading. This technique takes some practice, of course.
Conduct with the Left Hand
Now when you play with your feet and your right hand only, you can conduct with the left hand. Simply use whatever scheme you need to conduct the meter properly. Use your free hand to show choir entrances and stops also. You can even make some dynamics with your free hand. If your movements are small, your choir will sing softly. To achieve a stronger sound, use wider movements. Make sure that you point to the correct portion of your choir if only a part of it is entering at any particular spot.
Practice Conducting and Playing at the Same Time
If you try to conduct and play at the same time, you will notice right away how tricky it might be. It is very easy to hit the wrong notes or start conducting incorrectly or both. This happens because your hands must accomplish very different tasks. In other words, you need to achieve hand coordination. Perhaps even more importantly, your brain must also do the same. In order to achieve the fluency while conducting and playing at the same time, you will need to practice your movements. Practice your hymns or anthems ahead of time. Repeat the small sections as many times as you need to do them correctly at least three times in a row. Then combine the shorter fragments into longer episodes.
The practical techniques of accompanying the choir with or without a conductor are discussed in Organ Technique: Modern and Early by George Ritchie and George Stauffer which I highly recommend.
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