I noticed myself doing three visibly different things to the same paragraph in the same sitting last week and wanted to write down what each one actually meant, because I don’t think I’d ever separated them before.
Underlining a sentence means: this is well made, I want to be able to find it again, and I have nothing to add. It’s an act of preservation, not response. A marginal note, by contrast, almost always means I disagree, or think the author has skipped a step — the note is doing work the text isn’t. And the dog-ear, the humblest of the three, just means come back to this page, without committing to why.
What’s useful about naming the distinction is noticing how often I default to underlining when what I actually mean is disagreement, because underlining is faster and doesn’t require having a formed thought yet. Which means my books are full of sentences marked as admired that I actually, on inspection, half-disagreed with — a smaller version of the problem discussed in this note about margins collapsing the space between reading and responding. Sometimes the collapse goes the wrong way and just files the disagreement as praise.