Multiple voices on a staff
Spine split manipulators (*^) and spine merge manipulators (*v) are used
to add or remove voices/layers on a staff. Note that the highest part
on the staff will typically be left most (opposite of the staff ordering
from lowest to highest).
Notes sounding together in the different voices are placed on the same line, and
if the other voice is sustaining, a . character is used as a place holder for
that voice (the dot is called a null token in Humdrum terminology).
Partial voices/layers
Voices/layers which do not continue throughout the measure can be encoded in two equivalent manners: (1) splitting and merging the spine as needed in the measure, or (2) adding invisible rests to fill in the voice/layer for the entire measure. For method #1, note that the spine merging cannot occur until the end of both sounding notes in each layer.