[Matt Q]

Turco also uses 'x' for describing (syllabic) forms with internal rhymes. To choose a simpler example, the Welsh bardic form Englyn Cyrch has quatrains like this:

x x x x x x a
x x x x x x a
x x x x x x b
x x x x b x a

using an x makes a lot of sense in situations like this, where there are so many non-rhymes. Compare the non-X method:

a b c d e f g
h i j k l m g
o p q r s t u
v w z y u z g

where the rhyme scheme becomes very hard to see (and you risk running out of letters!)