When a four-rank mixture is drawn and a key played,
four pipes sound. A mixture usually supplies HIGH PITCHES that are
used in combination with lower-pitched stops that must be drawn independent
of the mixture.
FOR EXAMPLE:
The Plein Jeu IV on the stop list to the right would not usually
be drawn without the support of the Bourdon 8', the Prestant 4',
and the Blockflöte 2'.