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I have always pronounced the th in "posthumous" as if it was the "th" in think (/θ/), but when I searched itd it was actually the ch /tʃ/:

  • UK: /ˈpɒs.ə.məs/
  • US: /ˈpɑːs.ə.məs/

I found a language log discussion, but it has not explained it.

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    I always pronounce the first 7 letters like those of 'lost human', with the emphasis on the first syllable. – Michael Harvey Jan 7 at 13:02
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    I think I pronounce /tj/, but I can see how it could be /tʃ/ for some speakers. The "h" is silent. – rjpond Jan 7 at 13:07
  • @rjpond Texas here, and the h most definitely is not silent; there's unvoiced air while forming the "o" vowel. – chrylis -cautiouslyoptimistic- Jan 8 at 9:17
  • I pronounce it as 'post - hummus' like after you eat some ground chickpea paste, seasoned with tahini (sesame paste), minced garlic, and extra virgin olive oil. – Mitch Jan 8 at 14:04
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TL;DR

The reason why the ⟨th⟩ in posthumous is pronounced /t͡ʃ/ (ch) is the coalescence/assimilation1 of the t and the following u.

Explanation

'Posthumous' is made up of the prefix post- and humous. Post ends in a /t/ and the ⟨h⟩ in humous is silent so it starts with a u which is basically /juː/ (the same as the u in 'cue'). We could say that humous starts with a glide /j/.

  • post + (h)umous

the letter u is /juː/:

  • pos/t/ + /juː/mous

In fact, Lexico gives /ˈpɒstjʊməs/ for posthumous i.e. /t/ + /j/.

You might have noticed that when a /t/ comes before a /j/, there's a tendency to assimilate them to /t͡ʃ/ ('ch' as in chip). The /t/ is normally articulated at the alveolar ridge, but when it comes before a /j/ (which is articulated further back in the mouth—at the hard palate), it's usually pronounced /t͡ʃ/. What happens here is that the /t/ is articulated further back in the mouth in anticipation of the following /j/, so it becomes /t͡ʃ/ i.e. they coalesce/assimilate to a /t͡ʃ/:

  • pos /t͡ʃuː/ mous

That's where the /t͡ʃ/ came from. The same goes for the tu in nature (na/t͡ʃ/re).


1. Assimilation is a process that makes nearby sounds more similar to each other. The kind of assimilation in posthumous is called 'coalescent assimilation'. The following sounds often coalesce:

  • /t/ and /j/ coalesce to /t͡ʃ/ (as in posthumous)
  • /d/ and /j/ coalesce to /d͡ʒ/ (as in education)
  • /s/ and /j/ coalesce to /ʃ/ (bless you is sometimes pronounced bleshoo)
  • /z/ and /j/ coalesce to /ʒ/ (as in vision)

/t/ is the T in time
/j/ is the Y in you
/t͡ʃ/ is the CH in chin
/d͡ʒ/ is the J in join
/ʃ/ is the SH in ship
/ʒ/ at the end of massage

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  • Another relevant comparison for that tu assimilation is posture. – PLL Jan 8 at 9:31
  • I think you get the same effect with “posthumanism”, where you'd usually expect the ‘h’ to be pronounced as [ç] rather than completely silent. – James Wood Jan 8 at 11:39
  • Same effect with postulate. – Suncat2000 Jan 8 at 17:08
  • @Suncat2000: Mature, prostitute, substitute, posture, institute, astute, tune etc. (at least in BrE) – Void Jan 8 at 17:10
  • Just adding: Tudor. – Flater Jan 9 at 15:46
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'Post' is a common prefix in English, from the Latin for 'last' which to us essentially means 'after'.

The English 'posthumous' literally means after burial. For the full etymology of the word from Latin, see this reference.

Some words that use this prefix are not compound words but hyphenated, for example, 'post-mortem' (an examination after one's death).

So the reason why the 'th' are not pronounced together as in the word 'think' is because they are in effect part of different words - the 't' is the end of the prefix and the 'h' is the beginning of the root word. A comparable example would be the word 'lighthouse' - a compound of 'light' and 'house'.

However, the 'h' isn't really pronounced at all in 'posthumous'. There are plenty of other words where an 'h' following a 't' is not pronounced - such as neanderthal, thyme. These do seem to be exceptional though, as 'h' is pronounced more often than not. Etymology is very often the reason, and unfortunately, there is no 'rule' to guide you on how to pronounce 'th' in an unfamiliar word.

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    @rjpond it appears to have changed over time - I'm not getting into that as the definition isn't part of the question anyway, so I've removed that and added a link. – Astralbee Jan 7 at 13:09
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    But why is the th pronounced /t͡ʃ/? – Void Jan 7 at 13:09
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    @Void the "th" isn't pronounced /t͡ʃ/. The "thu" is /tju:/ (the "h" is silent), which, due to yod-coalescence, can become /t͡ʃu:/. – rjpond Jan 7 at 13:11
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    @rjpond: I know! I'm in the middle of explaining it. But this answer doesn't explain that – Void Jan 7 at 13:11
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    @HagenvonEitzen I accept that neanderthal is a toponym, but ,,, thyme... really? – JavaLatte Jan 8 at 6:55

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