Good catch. In fact, it's possible that TeX (and humans) judge the first paragraph as better. Here they are, with colons after each: xxxx x xxxx xxxxx xx x

Good catch. In fact, it's possible that TeX (and humans) judge the first paragraph as better. Here they are, with colons after each: xxxx x xxxx xxxxx xx xx xxxxx x: xx xxxxxxx xxxxx xxx xxxx xxx : xxxxx x x xxx xxxx xxx xx xx : xxxxxxx x xxxx xx xxxx xxx xxx : xx xxxx x xxxx xxxxx xx xx xxxxx : x xx xxxxxxx xxxxx xxx xxxx xxx: xxxxx x x xxx xxxx xxx xx xx : xxxxxxx x xxxx xx xxxx xxx xxx : xx Note that in reality the paragraphs will not be typeset as above, but the spaces will be stretched or shrunk so that the lines have identical width. The reason the first could be judged as better is that TeX has penalties (\adjdemerits) when consecutive lines belong to "incompatible fitness classes" (e.g. very loose lines following very tight lines), so it may be possible that the first paragraph can count as (tight, loose, loose, loose), and the latter as (loose, tight, loose, loose), which is worse.