Line length and leading are set in angle, not physical length

What it is

Print’s “fifty-five to seventy-five characters a line, leading around 1.2 to 1.5 times the type size” is written in centimetres on paper. The same sixty-centimetre-wide paragraph at one metre and at three metres is not the same reading measure on the eye. Line length and leading are set in angle: how many degrees a line spans in the field, how many arcminutes sit between lines — not how many centimetres a tape reads on the panel. The leaf is those two typographic measures. It is not about scaling the type itself with distance.

Why it happens

Reading eye movements run in visual angle: a forward saccade covers some number of degrees, and the return sweep from line-end to next-line start is also settled in angle. Physical measure scales on the retina with distance, so “sixty centimetres a line” can be so wide nearby that it takes several large saccades to finish, and so narrow far away that each line holds a handful of words. Leading is the same: 8 mm on the panel can feel like air nearby and fall into the crowding zone at range, with the next line’s tops colliding with this line’s descenders.

Setting both in angle keeps saccade size and crowding distance in the same family across depths. A line that is too wide in angle drops the return sweep on the wrong row; a line that is too narrow spends the reader on wrapping. Leading below an angle-determined floor fuses adjacent lines into a texture. Centimetres are downstream of angle here: when depth changes, the centimetres must change so the angle is the end that is kept.

Studying it

The same passage, compared under “locked physical measure and leading” versus “locked line visual angle and inter-line arcminutes,” at two depths. Measure reading rate, return sweeps that land on the wrong line, and the subjective “line too long / lines packed.” Eye tracking can show whether forward-saccade amplitude jumps with depth.

Independent variables: depth, measure rule (centimetres locked / angle locked), leading rule, characters per line. Dependent variables: reading rate, wrong-line landings, saccade-amplitude distribution, crowding ratings.

Changing sitting distance in front of a paper print mixes accommodation and print resolution, which is not the headset’s bottleneck. Change panel depth in the headset and hold typeface and copy fixed.

Where it stops holding

A one-line label or two words on a button have no measure problem; the angle rule does not apply. Verse, code, and monospaced tables that align on physical columns will lose that alignment if the layout is rewritten to hold angle. On a narrow field of view the largest line angle available may not fit fifty-five Chinese characters, so the options are fewer characters or living with return sweeps. A huge environmental sign may span tens of degrees on purpose; it is there to be recognised, not read, and leading rules do not apply.

Applying it

• Give body text a target line angle (for example, finish a line inside roughly twenty degrees) and a minimum inter-line arcminute floor. When the panel moves, change the centimetres to keep those two angles.
• Do not paste a print “twelve centimetres per line” into the scene. First measure how far the eye must travel to finish a line at the intended stance.
• Do not share one angular budget between Chinese characters per line and Latin characters per line — Chinese carries more per glyph, so the same angle holds fewer characters.
• How to check: read the same passage at 1.2 m and 2.4 m. The angle-locked set should be close in wrong-line landings and rate; the centimetre-locked set should show “too long or too packed” at one of the depths. If both depths feel fine on the same centimetres, the depths have not been pulled far enough for the rule to fail.

Related

• Same group: N3.04.1 Tilt sharply reduces readability · N3.04.2 Text must face the user · N3.04.3 Complex backgrounds need a backing plate · N3.04.4 Minimum readable size is jointly set by angular resolution and pixel density · N3.04.5 Subpixel rendering fails under binocular viewing and motion · N3.04.7 Text that jitters with head motion is harder to read than still text
• Nearby: N3.02 Interface Distance · N3.07 Content at the Edge of the Field of View
• Search terms: angular typography · visual-angle measure · leading in arcminutes