Is there a rule whether a compound word is hyphenated (at a line break) between its components?

potash is hyphenated as pot-ash, but potassium is hyphenated as po-tas-si-um.
From Merriam Webster:

New Latin, from potassa potash, from English potash

Why is potassium not hyphenated pot-as-si-um, which would preserve the sense?

Is there some general rule about this?

  • The two "o"s are not even pronounced the same. What leads you to believe they should be hyphenated the same?
    – J.R.
    Oct 11, 2013 at 17:54
  • 2
    This question appears to be off-topic because it is about typesetting Oct 11, 2013 at 17:58
  • 2
    @FumbleFingers How is typesetting English off-topic? Hyphenation rules have a linguistic basis. Oct 11, 2013 at 19:29
  • I think potassa is treated as opaque, even if it did come from pot-ash.
    – user230
    Oct 12, 2013 at 4:00

1 Answer 1


Aside from breaking at double letters, hyphenation generally follows syllable boundaries. Potash is pronounced as the two words pot ash, but potassium is pronounced po-TASS-ee-um.

As a more specific rule, compound words, which until recently were often written hyphenated even in the middle of a line (mail-box, pot-ash), have a (preferred) hyphenation point between the base words.

  • The maximal onset principle tells us that /s/ belongs to /sɪ/, though you could argue that it's ambisyllabic (belongs to /ˈtæs/ as well).
    – user230
    Oct 11, 2013 at 20:02
  • @snailboat As a general rule I'd agree, but at least in American pronunciation, the Latin -ium suffix is such a chunk unto itself that a speaker speaking by syllable will almost always pronounce -ee-um as a standalone pair, attaching any consonant phonemes to the preceding syllable. Oct 11, 2013 at 20:05
  • Do I understand this right? 1. A word, which is recognized as and pronounced as a composition of two words, is hyphenated between the words, but a word, which is no longer recognized as composition and pronounced as if it was just one single word, is hyphenated according to the usual rules without respect to its constituent parts. 2. "potassium" falls into the second category.
    – Stephen
    Oct 13, 2013 at 13:09
  • @Stephen In my experience, yes, (1) is true. I know with certainty that (2) is false; it is a neo-Latin word made up in 1808. Oct 13, 2013 at 14:41

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