The phrase "a marginal note" is correct and usable in written English.
It can be used to refer to a note or comment written in the margin of a book or document, often to provide additional information or commentary.
Example: "As I read through the text, I found a marginal note that explained the author's intent more clearly."
Alternatives: "a side note" or "a margin comment".
Another reader had come to Folk's defense in a marginal note.
In a neat feminine hand, there was a marginal note: "Who is this man?" View Article By Malcolm Gladwell By Rivka Galchen By David Remnick By Jelani Cobb.
It was recently announced by the university library in Heidelberg that a printed book in its possession contains a marginal note, handwritten in October 1503, confirming that Leonardo was at work on a portrait of Lisa del Giocondo.
On a list of forty-five heavy-drinking writers, Bon had inserted his own name between Berryman and Boswell, appending a marginal note: "At what age did these die?
The assumption was verified in 2005 when a librarian at the University of Heidelberg, preparing a copy of the 1477 edition of Cicero's Epistoles ad Familiares for an exhibition, came upon a marginal note by Agostino Vespucci comparing Leonardo with Apelles, in which he notes that Leonardo was then working on a portrait of Lisa del Giocondo.
Almodóvar had his dialogue translated into English, and the results were tentative: "I had everything ship shape (or some local expression)." When Juliet went out on the street in Boston and smiled at everyone, he added a marginal note: "Consider if Boston society would naturally accept such Mediterranean behavior as that of being openly looked at in the street".
On page 1512, next to Hebrews 1 3, the text contains an interesting marginal note, "Fool and knave, leave the old reading and do not change it!" – "ἀμαθέστατε καὶ κακέ, ἄφες τὸν παλαιόν, μὴ μεταποίει" which may suggest that unauthorised correcting was a recognized problem in scriptoriums.
It was published shortly after the death of Leicester's son, which is alluded to in a stop-press marginal note: "The children of adulterers shall be consumed, and the seed of a wicked bed shall be rooted out".
I've escaped with not much worse than a few marginal notes along the lines of: "Can't you get a new coat?" or "You could look great with a makeover Prof Beard", all fairly friendly banter - though, the fact is, if I wrote something like that about them on a student report form I'd almost certainly get a ticking off.
But the author of the marginal note, a friend of Leonardo's called Agostino Vespucci, was certainly prompted to write in the margin by the notion that Apelles left things unfinished.
To which I responded with my most plentiful marginal note: "not a geek".
