Commencing with a C-shaped paraph-sign in the same brown ink, the glosses are linked to their corresponding passages by alphabetic signs comprising single letters in a sequence beginning with a in each margin, moving on to b, c, and d down the page, and, ideally, linking with the same letter in the adjacent column. Such a pairing allows for matching the gloss with the specific location in the text where its comment should be considered.
Source: wiktionary