Liturgical custom is a great joy.
Some things are done in the liturgy because of "custom" (ex consuetudine...) or "traditional usage" (ex instituto). As any Thomist knows, strict legalism is not Catholic. So the Catholic liturgical reach includes ex more (that which is habitually/acording to custom) because consuetudo pro lege servatur (custom is held as law).
Here are some Rome lines that reflect liturgical custom:
Mos majorum (ancestral custom), nil consuetudine majus (nothing is greater than custom), ad usum (according to custom), mores (customary usages/unwritten usages), mos pro lege (usage has the force of law).