Chiasmus?
This is not how a literature professor would explain the term, but what else would you call these crossed alphabets? Pontificale Parisiense. …
Gathering the alphabet
Horae ad usum romanum. Source: gallica.bnf.fr Bibliothèque nationale de France, Département des manuscrits, Latin 1156 B, fol. 135r Horae ad usum romanum. …