> Tighten wording. Document conventional space width. Fully discuss
Regarding the conventional space width, this patch added this parenthetical: > current font's space width (usually one-third em for Western scripts) But this claim is not reflected in the data for the fonts shipped with groff for the two most common typesetting devices, ps and pdf. $ fgrep unitwidth font/devp*/DESC.in font/devpdf/DESC.in:unitwidth 1000 font/devps/DESC.in:unitwidth 1000 So for these devices, 1/4 em would be 250 units, and 1/3 would be 333. The actual space widths for the fonts (pdf inherits the metrics from ps, I believe) are: $ fgrep spacewidth font/devp*/* | sort -k2 font/devps/ZCMI:spacewidth 220 font/devps/SS:spacewidth 223 font/devps/HNB:spacewidth 228 font/devps/HNBI:spacewidth 228 font/devps/HNI:spacewidth 228 font/devps/HNR:spacewidth 228 font/devps/PB:spacewidth 250 font/devps/PBI:spacewidth 250 font/devps/PI:spacewidth 250 font/devps/PR:spacewidth 250 font/devps/S:spacewidth 250 font/devps/TB:spacewidth 250 font/devps/TBI:spacewidth 250 font/devps/TI:spacewidth 250 font/devps/TR:spacewidth 250 font/devps/AI:spacewidth 277 font/devps/AR:spacewidth 277 font/devps/HB:spacewidth 278 font/devps/HBI:spacewidth 278 font/devps/HI:spacewidth 278 font/devps/HR:spacewidth 278 font/devps/NI:spacewidth 278 font/devps/NR:spacewidth 278 font/devps/ZD:spacewidth 278 font/devps/ZDR:spacewidth 278 font/devps/AB:spacewidth 280 font/devps/ABI:spacewidth 280 font/devps/NB:spacewidth 287 font/devps/NBI:spacewidth 287 font/devps/BMI:spacewidth 300 font/devps/BMR:spacewidth 320 font/devps/EURO:spacewidth 333 font/devps/BMB:spacewidth 340 font/devps/BMBI:spacewidth 340 font/devps/CB:spacewidth 600 font/devps/CBI:spacewidth 600 font/devps/CI:spacewidth 600 font/devps/CR:spacewidth 600 Disregarding the four monospace fonts, which require their own spacing rules, 29 of those fonts are closer to 1/4 em, and 5 of them to 1/3 em (though few are exactly either 1/4 or 1/3). This aligns with what our friend Heraclitean River (http://web.archive.org/web/20171217060354/http://www.heracliteanriver.com/?p=324) observed in his research: "Margins became smaller [in the first half of the 20th century], and standard interword spaces often went from about 1/3 em to 1/4 em." Incidentally, where the same parenthetical phrase is inserted into groff_diff(7), it introduces mismatched parentheses (not obvious as they go three levels deep).
