From A Place for Everything, by Judith Flanders (Basic Books, 2020), Kindle pp. 55-57:
In Europe, Isidore’s innovative use of alphabetical order to organize his work on vocabulary was instead proving influential in a parallel genre: glosses. Glosses had originally appeared in the manuscripts they were glossing, where they explained the meanings of difficult words in Latin or translated foreign words. In both cases, the gloss was written either above the relevant word or beside it, in the margin. Later, sometimes for clarity, sometimes to preserve a valuable manuscript, glosses began to be written out as a separate document, initially continuing to list the words in the order in which they appeared in the primary text, so tying a gloss to a single work, or even, because of the reshaping and reordering that we have seen in copied manuscripts, to one particular copy of a work. Gradually, however, the utility of a gloss that included vocabulary from more than one work became apparent, even if it meant it was no longer practicable to list the words in order of appearance.
And so experiments began with different ordering systems. One possibility was alphabetical order. Another was subject categories, particularly for glosses of technical vocabulary, grouping together words relating to hunting, for example, or words for military fortifications or for parts of the body. Other glosses relied on organizing principles that are far more foreign to us today. The Læcboc, or Medicinale anglicum, The Leech Book, or English Remedies, written in Old English around 950, was arranged, as were many medical texts of the period, with diseases and cures situated along the human body a capite ad calcem, from head to heel. In the Byzantine Empire, texts were generally organized by subject, sometimes geographically, or by name. Only the Suda, a Byzantine encyclopedic dictionary dating from the late tenth century, was an alphabetical compilation, magisterially ordering thirty-one thousand historical, biographical, and lexicographical entries into a single alphabetical order. But it was an outlier, both in Byzantium and in Western Europe too at that date.
Glosses, owing their existence to readers’ difficulties with Latin, were more common in countries where the local language had no etymological connection with Latin. Native English, or German, or Flemish readers had greater need of assistance with Latin vocabulary than did French or Italian or Spanish readers, and so it follows that some of the earliest glosses we know of, from the seventh and eighth centuries, were produced by English and Irish speakers. The Leiden Glossary was probably compiled in St. Gallen, but by someone who, judging from the English translations, probably came from what is now Kent, in England. He translated the Latin vocabulary into either Old English or Old High German, and arranged the entries, at least in part, in first-letter alphabetical order. By the eighth century one glossary, which defined nearly five thousand Latin words, ordered just under two thousand of them into fairly consistent first-letter alphabetical order. One extraordinary copy of another glossary, the Liber glossarum, The Book of Glosses, which may have been produced at a convent at Soissons and was based in part on Isidore’s Etymologies, was in almost absolute order, one of the very earliest examples.