Kent Zacherl asks [100225 MMDigest] how the 20-note scale is broken
down, other than the bass notes.  The usual split between the melody and
accompaniment sections is that there are 11 melody notes.  Of course,
with such a limited scale it is usual for melodies to encroach well
below the 11th note (counting from the highest) so there is no real
defined split.

Small organs have just a single rank of 20 pipes, whereas larger organs
may have multiple ranks throughout the scale.  I know of people who
have made their own organs, and arrange their own music, who have
multiple complete ranks of pipes which give a very effective rendition
of classical music.  Most commercial organ builders, however, stick to
an 11 note melody where multiple ranks are involved.

Nicholas Simons, GB