/t/ → /ʧ/
This is a very interesting case in which the sound /j/ is involved. The alveolar /t/ followed by the palatal /j/ turns into the post-alveolar /tʃ/. So, the place of articulation of the assimilated sound is neither that of the first phoneme nor that of the second: it’s a third one. The manner of articulation is also slightly different: affricate instead of plosive (as you know, affricates begin as plosives and continue as fricatives). This type of assimilation is called coalescence (to coalesce means to come together, merge, etc.) because two sounds fuse into a new phoneme in a place which isn’t that of either of them. We speak about coalescence or yod coalescence (yod is the name of the sound /j/ in the Hebrew alphabet) when the assimilation process involves the phoneme /j/ (we’ll see four different cases).
/t/ (followed by /j/) becomes /tʃ/
Listen to these first two recordings. I’d like to stress the fact that this doesn’t come from a bunch of friends having a drink in a pub, but from what is probably one of the most perfect instances of standard, well-spoken English: a BBC News bulletin.
And here are some examples of the same type:
Josh Surtees is a journalist who writes for The Guardian and lives in Trinidad and Tobago. Last year he received an email from our Middle East correspondent, Bethan McKernan (podcast from The Guardian).
What is needed, though, is that you understand the mental demands of those sports and are able to adapt your work, so that it can be integrated into the performance environment. But if you’re not honest to your clients about what you realistically can and can’t do, you won’t progress very far (from a Cambridge Advanced test).
One very important fact is that quite often this process takes place within a single word:
He was paraded in the École normale, in the École militaire, in the huge courtyard, and he was stripped, ceremonially, stripped of all his epaulettes and decoration and his sword was broken in front of him and he was sent off to, as you said, Devil’s Island (Robert Gildea, BBC4). /ˈkɔːtjɑːd/ → /ˈkɔːtʃɑːd/
In this period, the papal incomes shifted rather from being mainly derived from being spiritual incomes (Catherine Fletcher, BBC4). /ˈspɪrɪtjuəl/ → /ˈspɪrɪtʃuəl/ In this case, the assimilated version is, of course, the most common one.
In some words, the sound /tʃ/, historically derived from /t/ + /j/, has become the only possibility. This is what happens, for example, with the word nature.
Listen to the pronunciation of the words nature and perpetual in this recording:
Nature is always pronounced /ˈneɪtʃə/. Perpetual is almost always pronounced /pəˈpetʃuəl/. The pronunciation /pəˈpetjuəl/ is possible but quite rare.