Field notes
Everything else in a long article is negotiable. The measure is not. It decides whether the eye finds the start of the next line without effort, and a reader who loses that line twice does not come back for a third.
Set a paragraph across a full desktop window and you have built a wall. The eye reaches the right-hand edge, sweeps back, and lands a line above or a line below the one it wanted. Nobody notices this happening to them. They notice only that the article felt like work, and that they gave up somewhere in the middle of it.
The remedy was settled long before screens. Hold the line somewhere between forty-five and seventy-five characters and the return sweep becomes automatic again, which is another way of saying it becomes invisible.
Note 01
Forty-five to seventy-five characters, spaces counted. Two-thirds of that is a fair target for a column standing beside something else.
The awkward part is that a measure cannot be built from the units most layouts are built from. A width expressed as a share of the window moves when the window moves, and the type does not move with it, so the number of characters on a line drifts every time somebody drags a corner.
A measure has to be tied to the size of the thing it is measuring. State it in the same units as the type, and the line holds its character count in a narrow window, in a wide one, and on a display nobody has looked at yet.
Note 02
The reading column here is stated in rem, not in a share of the viewport. The letters and the column grow together, so the count does not move.
What is left over is the interesting part. Once the text stops at a fixed measure there is a strip of quiet page beside it, and that strip is not waste. It is where everything that would otherwise interrupt the argument can go and wait.
A definition, a caveat, a source, a second thought. Set any of them inside the paragraph and the sentence stalls while the reader carries the parenthesis to the end of it. Set them in the margin and they cost nothing until they are wanted.
Note 03
A note in the margin is optional reading. The same sentence in the paragraph is compulsory, and it costs the reader the thread.
Marginalia only works while there is a margin. On a phone there is none, so a note has to become something else rather than vanish, and rather than be swept to the foot of the article hundreds of words away from the sentence it belongs to.
So it settles directly beneath the paragraph it annotates, under its own rule, still set smaller than the body. The geometry is gone and the relationship is not, which is the only part of it a reader was ever reading.
Note 04
Every note is tied to its own paragraph rather than to the article. That is what lets the rail collapse without scattering.
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