I found that rule on one of the sites related to learning English and I would like to know whether the information is correct. The source is not in English so I will try to translate it. If a noun ending with -th has a long vowel sound or a diphthong before this ending,we have the following changes in the plural: [θ] is changed for [ ð], and –s is pronounced like [z]: a path [pɑːθ] – paths [pɑː ðz]; a mouth [mauθ] – mouths [mau ðz].

Is it always true or can we also use a general rule with such words and pronounce -s voicelessly?

  • In thinking about it for a minute or two I couldn't think of any exceptions, but that doesn't mean there aren't any. Note that the pronunciation of the "th" in such words changes in a similar way if you add "ing", as in "mouthing".
    – nnnnnn
    May 21 '16 at 4:54
  • Except many/most native speakers of American English pronounce clothes as /kloʊz/ dropping the /θ/ (/ð/) altogether. Other speakers do say, or attempt to say, /kloʊðz/. Many just think they're saying that. May 21 '16 at 5:39

This is a pattern that used to exist in many words in English a long time ago. It still occurs with many words in English, but it is not a rule. Some th-plurals are almost always pronounced with /ðz/, for example mouths. Some are frequently pronounced with either /ðz/ or /θs/, for example paths (in British English). Some are nearly always pronounced just like a regular plural with /θs/, for example the word faiths.

It won't be considered wrong if you say any of these words with /ðz/, but native speakers will often use /θs/ for many of these words.


On a list of most frequently used English nouns, I found growth, earth, youth, birth and faith, at least some of which (for me, at least) always (growths, earths [as in 'Astronomers searching for exo-planets have found many Jupiters but no Earths'], births and faiths) or sometimes (/juθs/ or /juðz/) retain the /θ/. I think these are exceptions because they were originally uncountable nouns, and were more lately used countably.

  • 1
    For me (most-or-less standard Australian English), yes. (I discarded some others.)
    – Sydney
    May 21 '16 at 14:17
  • Possibly some people with rhotic accents pronounce 'birth' and 'earth' differently, but as far as I know, 'growth' 'youth' and 'faith' have 'a long vowel sound or a diphthong' (per OP's question).
    – Sydney
    May 22 '16 at 0:26
  • Yes, you're right they certainly do! (have long vowels or diphthongs) May 22 '16 at 1:11

The general rule for pronunciation of s in plurals does apply to these words.

The general is that, if the last phoneme of the noun is voiced, the s is also voiced /z/ and if the last phoneme is unvoiced the s is also unvoiced /s/.

What's unusual about words like path and mouth is not the pronunciation of the s, which always follows the general rule, but that the th changes from unvoiced /θ/ in the singular to voiced /ð/ in the plural: this in turn causes the s to follow the general rule and change to voiced.

You get exactly the same effect with words ending with a long vowel followed by f: the unvoiced f in the singular becomes a voiced v in the plural. This is better documented because the spelling changes too: loaf -> loaves and hoof -> hooves.

Note that in regional accents where the pronunciation of the vowel is different, the outcome may be different. For example, in Yorkshire, Norhern England the a in path is pronounced as a short /æ/ as in cat: the th therefore remains unvoiced in the plural, and so the s also remains unvoiced- /pæθ/ -> /pæθs/.


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