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I was teaching my young nephew some math the other day, and from discussing the typical sort of word problems he's encountering in class, I noticed that the "-th" suffix adds a distinct meaning to adjectives. For example:

  • If a ship is long, it has length.
  • If a woman is wide, she has width.
  • If a person is strong, he possesses strength.
  • If what I say is true, I'm speaking truth.
  • A lumbering panda moving slow is full of sloth.

Now, I've learned some linguistics from English L&U, and I'm guessing this "-th" suffix is an affix that changes adjectives into nouns. My questions are: What exactly is this "-th" suffix adding to the meaning? Secondly, does the "-th" originate from a separate word in Old English? Lastly, is there something to say about the vowel shifts that seems to be occurring in some of the transformations (e.g., strong going to strength) that somehow fits in with the ablaut system of strong verbs/weak verbs, that I learned of from the excellent responses to my previous question?

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I upvoted your question, it's interesting :) And it would be so on the Linguistics.SE too! – Alenanno May 20 '11 at 11:28
Very interesting question! I am agreeing more and more with the usefulness of the Linguistics site. – Kit Z. Fox May 20 '11 at 11:31
+1 and for the record, I think -th sounds much better than -ness. -ness is quite overused, especially since there's often already a word available: why accurateness instead of just accuracy? Also consider the similar growgrowth, from OE -ð(u). – Jon Purdy May 20 '11 at 18:00
1. en.wiktionary.org/wiki/-th 2. Old English -ðu, , from Proto-Germanic -*itho, abstract noun suffix, from PIE -*ita (cf. Sanskrit -tati-; Greek -tet-; Latin -tati-, as in libertatem "liberty" from liber "free"). etymonline.com/index.php?term=-th 3 See also, books.google.co.in/books?id=aDhGlKL3h00C&pg=PA586 – Kris Jan 13 '14 at 12:38

It is, as you say, a nominalising suffix, that goes back at least to Common Germanic. It usually surfaces as "-th" in English, but as "-t" after a (historical) "-(g)h": "height", "weight", "sight", "flight".

The vowel alternations are mostly a matter of length (though I admit that the "-ong"/"-ength" alternation sounds a bit like IE ablaut). "Wide" in Old and Middle English was bisyllabic, with a long vowel, whereas "widþ" had one closed syllable and therefore a short vowel. Only the long vowel went waltzing round the mouth in the Great Vowel Shift. Thinking about it, this particular alternation may go back to a time when the IE ablaut was operating, but I'm not sure.

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Note that many do say heighth [haɪ(t)θ] or [haɪʔθ] for height, by analogy with those words that do have th, and since gh is null in modern English. I'd say weighth is less common, and for other -ght words it's hardly heard at all. People can complain all they want about it being incorrect, but hey, the only reason it was -t and not -th in the first place is gone, so why not? – Jon Purdy May 20 '11 at 18:08
I don't recall ever hearing that pronunciation, but the OED lists [haɪθ] and indeed the spelling 'highth'. – Colin Fine May 21 '11 at 0:32
@JonPurdy The four of us were dressed in the heighth of fashion, ... A Clockwork Orange ... BTW, I say heighth*/*highth in the singular but heights in the plural :shrug: – AnWulf Feb 20 '12 at 2:46

The purpose of the suffix -th is, yes, to make adjectives into nouns. It can also change verb actions into nouns as well i.e. 'grow' into 'growth'

This suffix actually came from an Old English letter, -þ, which actually also had the same function of turning verbs and adjectives into nouns.

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Idiot: Not that I'm doubting the truth of this, but it would be nice to see a corroborating reference. Such as merriam-webster.com/word/word.php?date=Apr-26-2010 – FumbleFingers May 20 '11 at 12:50
Rather, the suffix has had the sound /þ/ or variants thereof since Old English, when it was usually written "þ". A linguistic element or sound cannot come from a letter, which is merely a way of writing down a sound. – Colin Fine Feb 21 '12 at 0:52

Th suffix represents a continued on in the word example believeth opposed to just saying believe believeth means continue to believe,saith means continue to say

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That's an entirely different -th, nothing to do with this question at all. It's also wrong: ‘believeth’ is simply an older form of ‘believes’, there is no difference in meaning. – Janus Bahs Jacquet Jan 13 '14 at 10:13

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