Marginalia are marks or notations or illustrations drawn into (obviously) the margin of a document. They have another name: apostils, from the Middle French verb apostiller, meaning "to add marginal notes." This in turn was from Latin postilla, "little post." The origin of postilla might be (we aren't sure) the Latin phrase post illa, which would mean (if Latin used it this way) "after these things."
Scholarly works and the Bible would have marginalia such as numbers to denote divisions of texts, or notes for liturgical use. There may even be scholia, ("comment, interpretation") which are corrections in grammar or translation or comments on the text referencing other works. Errors could creep into the arduous task of copying, and a subsequent copier of the copy could be aware that a mistake had been made, which he would seek to correct with a scholia. The person who added scholia was a scholiast, a word that goes back to the 1st century CE.Additionally, a monk who had knowledge of a commentary on a document he was copying might decide to add scholia to offer an explanation on the particular passage in front of him. Modern book lovers debate over the propriety of writing in a book; these monks saw fit to "pre-write" into the work for clarification.
This is not to say that all scholia are to be trusted. Mistakes can be made. For example, there is a 1314 manuscript of a 3rd century text, Porphyry’s Homeric Questions (a discussion of problems that arise from reading the works of Homer). There are other manuscript copies of the works of Homer that have scholia that are clearly quoting Porphyry—and they are different. The person adding the scholia has mis-remembered the original; or did he? Maybe the person who made the 1314 copy was being sloppy while looking back and forth from the written to the being-written in front of him.
If I am in the position where I cannot digitally copy and paste, and must read + remember + type a longish passage, I must be extra careful because I know how easily my short-term memory can "smooth over" the original. I cannot imagine a monk would not try to speed up the tedious process of copying by spending less time shifting back and forth.
In the illustration above, a printed copy of Homer's Odyssey (printed in 1535), the printer Johann Herwagen (1497–1559?) has simply included (the narrower column) and additional scholia from other manuscripts, leaving the reader to decide which is preferred.
If the readers of this blog follow any other information about the Middle Ages, however, yesterday's reference to marginalia conjured images of, well, images. The phrase medieval marginalia usually makes people think of pictures, and I'll give you a representative sample of those next time.