Homeric scholarship is the study of any Homeric topic, especially the two large surviving epics, the Iliad and Odyssey. It is currently part of the academic discipline of classical studies. The subject is one of the oldest in education.
Scholia are ancient commentaries, initially written in the margins of manuscripts, not necessarily at the bottom, as are their modern equivalents, the notes. The term marginalia includes them. Some are interlinear, written in very small characters. Over time the scholia were copied along with the work. When the copyist ran out of free text space, he listed them on separate pages or in separate works. Today's equivalents are the chapter notes or the notes section at the end of the book. Notes are merely a continuation of the practice of creating or copying scholia in printed works, although the incunabula, the first printed works, duplicated some scholia. The works of Homer have been heavily annotated from their written beginnings.
The number of manuscripts of the Iliad is currently (2014) approximately 1800. The papyri of the Odyssey are less in number but are still in the order of dozens. The inventory is incomplete, and new finds continue to be made, but not all these texts contain scholia. No compendium has collated all of the Homeric scholia.
Following the Principle of Economy: the allocation of scarce publication space to overwhelming numbers of scholia, the compilers have had to make decisions about what is important enough to compile. Certain types, or lines, have been distinguished; scholia have lines of descent of their own. Eleanor Dickey summarizes the most important three, identified by letter as A, bT, and D.
A, "the Venetian scholia", are most of the scholia of Venetus A, a major manuscript of the Iliad, dated to the 10th century, and located in the Biblioteca Marciana (Library of St. Mark's) of Venice. The sources of the scholia are noted at the end of each book. There are basically four.