Sidenote

A sidenote is set in the outer margin of the text, level with the line it annotates, so the eye travels sideways rather than to the bottom of the page and back. That single change removes the whole footnote transaction: no jump, no return, no losing your place, and no numbered list at the end that a reader has to decide whether to visit. Edward Tufte is the reason most people on the web know the pattern by name, because his books put nearly all of their annotation in the margin and his layouts have been copied into stylesheets ever since. The idea is much older though, and the medieval gloss written beside a manuscript line is the same instinct.

The pattern only works if the page is built to leave the room. That means an asymmetric column: the text keeps a comfortable measure and the margin is a real, permanent band beside it rather than whatever space happens to be left over. A note is then positioned vertically against its reference, which is why sidenotes fight with anything that reflows the text independently of the margin, and why they are awkward inside text columns: with two or more columns of running text there is no single outer margin for a note to belong to. The reference marker in the text is usually a small numeral or symbol, and pairing it with a hanging indent on the note itself keeps the marker legible against the note’s own first line.

It is worth being precise about what a sidenote is not. A tooltip is summoned and transient, appearing where the pointer is and leaving when it goes, while a sidenote is always visible and positioned by its reference rather than by the reader’s cursor. That difference is the whole argument for the form: the annotation is part of the page’s composition, so it can be skimmed, skipped, or read in passing without any action at all, and it is present for someone printing the page or reading it without a pointer.

The hard part is narrow screens, where the margin does not exist. The usual answer is to let the note collapse back into the flow, right after the paragraph that refers to it, behind the same numbered marker used as a toggle. That keeps the note near its sentence, which is the point, even though the marginal position is gone. Two things to keep honest when you do this: the collapsed note must still be reachable and announced in the reading order, and the marker has to be a real control with a real target size, not a numeral scaled to eight pixels because it looked right in the wide layout.