Custom divisions of chord squares can be defined through the
measure-division-lines-alist and measure-division-chord-placement-alist
properties of ChordSquare. These are both alists. Their keys are
measure divisions, namely lists which give the fraction of the measure
that each chord (or rest, or skip) represents. More precisely, a measure
division alist is made of positive, exact numbers adding up to 1, for
example: '(1/2 1/4 1/4). The exactness requirement means that,
e.g., 1/2 is valid but not 0.5.
The values in measure-division-lines-alist are lists of lines,
which are represented as (x1 y1 x2 y2).
The line starts at the point (x1 . y1) and ends at
(x2 . y2). Coordinates are expressed in the
[-1, 1] scale relative to the extent of the square.
The values in measure-division-chord-placement-alist are
lists of (x . y) pairs giving the placement of
the respective chords.
This example defines a peculiar chord grid style that has a rule for measures divided in three equal parts.