First let's define some terms: ***Elision*** This involves the complete omission of individual sounds. It can happen at word boundaries or word internally. An example would be the omission of /d/ in the word *friendly*, so that it sounds like "frenly": - He's very friendly --> /hiz 'veri 'frenli/. This can happen word internally or at word boundaries. An example at a word boundary would be in the sequence "last September". Here the /t/ is likely to be missing - we get something like "lass september": - Last September --> /la:s septembə/ ***Assimilation*** This is an altogether different process. This is when two sounds occur next to each other. One of the sounds changes to a different phoneme and becomes more like the sound it's next to. In other words, it assimilates properties of the sound it's next to. So, for example, when we say the word *input*, the sounds that we are notionally putting in there are as follows: /ɪnpʊt/. However, that /n/ there before the /p/ is very unstable in English. In fact, behind your top teeth - you can feel it with your tongue - there's a little shelf kind of thing. That shelf there is called your *ALVEOLAR RIDGE*. All the sounds that we make using that shelf, including /n/, are very unstable in English. When /n/ occurs before another consonant, it tends to move to the same place in the mouth as that consonant. In this case /p/ is *bilabial*, it's made with the lips. The /n/ therefore becomes bilabial too. It remains a nasal sound, but it becomes a bilabial nasal. A bilabial nasal is absically an /m/. So, in normal speech we don't actually say "i**np**ut" we say "i**mp**ut". - Thanks for the input --> /'θæŋks fə ði 'ɪmpʊt/ Again this can happen word internally or across word boundaries: - Bir**d b**ath --> /bɜ:b ba:θ/. ***Coalescent assimilation*** This is really a quite different phenomenon from normal assimilation. Occasionally, when we have two sounds next two each other the adjustments we need to make to each one to be able do them in succession result in a third completely different sound which replaces the original two. One such example of this is when we have /z/ followed by /j/ (the sound represented by the letter 'y'). This can sometimes result in replacement 'fused' sound, /ʒ/ (the middle consonant in vision). So in the sequence "these yours" we occasionally get: - Are these yours --> /ə ði:ʒɔ:z/ Here /ʒ/ is acting both as the last consonant in "these" and the first in "yours". Arguably, we also get this word type of coalescence internally in words like "leisure" /'leʒə/. ***Ellipsis*** Ellipsis is when whole words or series of words are omitted from speech or writing. It's considered a grammtical phenomenon, not an issue of pronunciation or phonetics and phonology. One example of this is when Joey from the television sitcom Friends says: - How *you* doin? Here the auxiliary *are* has been ellipted from the sentence *How are you doin?*. ***The Original Poster's example phrases***. The Original Poster's question was about the phrases "what-cha" and "aren't-cha". These are forms of "what you" and "aren't you". In as much as "what-cha" ever occurs as a form of the phrase "what are you", for example in "what are you gonna do about it", it only occurs in sentences where the auxiliary *are* has been ellipted. In other words the whole word *are* is completely missing: watcha only occurs as a form of "what you gonna to do about it" Both of these phrases are examples of coalescent assimilation, not elision or regular assimilation. This particular type of coalescence is often called *yod coalescence*. When the sound /t/ occurs before the sound /j/ in normal speech there is a tendency for them to be replaced by the sound /tʃ/ as in **ch**in. In the sequences "what you" and "arent you" we find /t/ and /j/ at the junction of the two words: - /wɒt ju/ and /a:nt ju/ [British English] --> /wɒtʃu/ and /ɑ:ntʃu/ - /wət ju/ and /ɑrənt ju/ [General American] --> /wətʃu/ and /ɑrəntʃu/ In casual speech the /u/s at the end there mau turn into /ʃ/, which si the "whatcha" and "arentcha" in the OP's question. One might be tempted to think that the "what-cha" in American English comes from a co-articulation of "What are you ..." /wət ər ju/ where the /t/ in 'what' and the /r/ in "are" join together to give a post-alveolar apico affricate - which is very similar in sound to /tʃ/ (see John Lawler's comments under the original question). However, this is definitely not the case. There are two straightforward pieces of evidence. Firstly, this pronunciation occurs in British English in all the same environments, but British English doesn't have an /r/ sound in the word "are". Secondly the "ch" sound in "what-cha" doesn't occur in American English in phrases like "what are they ...". If it was the /t/ and the /r/ that were producing the "ch" sound, it would obviously occur here - but it doesn't. ***Yod coalescence in young people's speech*** The Original Poster was wondering why young people use yod coalescence, and what this says about their language development. Well, firstly it should be said that the /tʃ/ phoneme is one of the very last to be acquired by children. Children usually manage to master the following significantly earlier than they manage /tʃ/: - / p, b, t, d, k, g, m, n, ŋ, f, v, s, z, h, w, l, j/ So the occurrence of /tʃ/ is a welcome sign in a young persons phonemic inventory. More significantly, yod coalescence, although it is often thought of as "not proper" is in fact widespread across all standard varieties of English. It doesn't only occur with "wotcha" and "arencha" but commonly at boundaries where we find /tj/ sequences. Linked in here is a video of Her Majesty Queen Elisabeth II, of England. If you forward to the following stages of the video you will hear Her Majesty doing various "watchu"-type articulations: - 05.22 "It's quite an old fashioned idea **thatchu** do put out the red carpet - for the guest" - 07.00 "You hope **thacho** going to get the, the ... - 07.03 ... the answer **thatchu** want". - 09.15 "You know. You know exactly **whatcho** going to be doing to months hence" - 09.29 "You have to sort've work out in your mind the hard work, an then **whatchu** enjoy in retrospect ... " Video like here: [Her Majesty Queen Elisabeth II](https://www.youtube.com/watch?v=Lh1-9ww_HmM) The upshot of this is that, young people should not be made to feel bad about yod coalescing or "wotcha-ing", it's perfectly natural and healthy - as well as posh and boringly grown-up. The idea that yod coalecence is not a feature of standard Englishes is a myth promulgated by ne'er do well prescriptionists. And they don't like that: what art they going to do about it?