Here’s something peculiar I noticed the other day about the English language.
The word is (the third-person singular present indicative form of the verb be) can be ‘contracted’ with a preceding noun phrase, so that it is reduced to an enclitic form -‘s. This can happen after pretty much any noun phrase, no matter how syntactically complex:
(1) he’s here
(2) everyone’s here
(3) ten years ago’s a long time
/ˈtɛn ˈjiːəz əˈgəwz ə ˈlɒng ˈtajm/
However, one place where this contraction can’t happen is immediately after the proximal demonstrative this. This is strange, because it can certainly happen after the distal demonstrative that, and one wouldn’t expect these two very similar words to behave so differently:
(4) that’s funny
(5) *this’s funny
There is a complication here which I’ve kind of skirted over, though. Sure, this’s funny is unacceptable in writing. But what would it sound like, if it was said in speech? Well, the -’s enclitic form of is can actually be realized on the surface in a couple of different ways, depending on the phonological environment. You might already have noticed that it’s /-s/ in example (4), but /-z/ in examples (1)-(3). This allomorphy (variation in phonological form) is reminiscent of the allomorphy in the plural suffix: cats is /ˈkats/, dogs is /ˈdɒgz/, horses is /ˈhɔːsɪz/. In fact the distribution of the /-s/ and /-z/ realizations of -‘s is exactly the same as for the plural suffix: /-s/ appears after voiceless non-sibilant consonants and /-z/ appears after vowels and voiced non-sibilant consonants. The remaining environment, the environment after sibilants, is the environment in which the plural suffix appears as /-ɪz/. And this environment turns out to be exactly the same environment in which -’s is unacceptable in writing. Here are a couple more examples:
(6) *a good guess’s worth something (compare: the correct answer’s worth something)
(7) *The Clash’s my favourite band (compare: Pearl Jam’s my favourite band)
Now, if -‘s obeys the same rules as the plural suffix then we’d expect it to be realized as /-ɪz/ in this environment. However… this is exactly the same sequence of segments that the independent word is is realized as when it is unstressed. One might therefore suspect that in sentences like (8) below, the morpheme graphically represented as the independent word is is actually the enclitic -‘s, it just happens to be realized the same as the independent word is and therefore not distinguished from it in writing. (Or, perhaps it would be more elegant to say that the contrast between enclitic and independent word is neutralized in this environment.)
(8) The Clash is my favourite band
Well, this is (*this’s) a very neat explanation, and if you do a Google search for “this’s” that’s pretty much the explanation you’ll find given to the various other confused people who have gone to websites like English Stack Exchange to ask why this’s isn’t a word. Unfortunately, I think it can’t be right.
The problem is, there are some accents of English, including mine, which have /-əz/ rather than /-ɪz/ in the allomorph of the plural suffix that occurs after sibilants, while at the same time pronouncing unstressed is as /ɪz/ rather than /əz/. (There are minimal pairs, such as peace is upon us /ˈpiːsɪz əˈpɒn ʊz/ and pieces upon us /ˈpiːsəz əˈpɒn ʊz/.) If the enclitic form of is does occur in (8) then we’d expect it to be realized as /əz/ in these accents, just like the plural suffix would be in the same environment. This is not what happens, at least in my own accent: (8) can only have /ɪz/. Indeed, it can be distinguished from the minimally contrastive NP (9):
(9) The Clash as my favourite band
In fact this problem exists in more standard accents of English as well, because is is not the only word ending in /-z/ which can end a contraction. The third-person singular present indicative of the verb have, has, can also be contracted to -‘s, and it exhibits the expected allomorphy between voiceless and voiced realizations:
(10) it’s been a while /ɪts ˈbiːn ə ˈwajəl/
(11) somebody I used to know’s disappeared /ˈsʊmbɒdiː aj ˈjuːst tə ˈnəwz dɪsəˈpijəd/
But like is it does not contract, at least in writing, after sibilants, although it may drop the initial /h-/ whenever it’s unstressed:
(12) this has gone on long enough /ˈðɪs (h)əz gɒn ɒn lɒng əˈnʊf/
I am not a native speaker of RP, so, correct me if I’m wrong. But I would be very surprised if any native speaker of RP would ever pronounce has as /ɪz/ in sentences like (12).
What’s going on? I actually do think the answer given above—that this’s isn’t written because it sounds exactly the same as this is—is more or less correct, but it needs elaboration. Such an answer can only be accepted if we in turn accept that the plural -s, the reduced -‘s form of is and the reduced -‘s form of has do not all exhibit the same allomorph in the environment after sibilants. The reduced form of is has the allomorph /-ɪz/ in all accents, except in those such as Australian English in which unstressed /ɪ/ merges with schwa. The reduced form of has has the allomorph /-əz/ in all accents. The plural suffix has the allomorph /-ɪz/ in some accents, but /-əz/ in others, including some in which /ɪ/ is not merged completely with schwa and in particular is not merged with schwa in the unstressed pronunciation of is.
Introductory textbooks on phonology written in the English language are very fond of talking about the allomorphy of the English plural suffix. In pretty much every treatment I’ve seen, it’s assumed that /-z/ is the underlying form, and /-s/ and /-əz/ are derived by phonological rules of voicing assimilation and epenthesis respectively, with the voicing assimilation crucially coming after the epenthesis (otherwise we’d have an additional allomorph /-əs/ after voiceless sibilants, while /-əz/ would only appear after voiced sibilants). This is the best analysis when the example is taken in isolation, because positing an epenthesis rule allows the phonological rules to be assumed to be productive across the entire lexicon of English. If such a fully productive deletion rule were posited, then it would be impossible to account for the pronunciation of a word like Paulas (‘multiple people named Paula’) with /-əz/ on the surface, whose underlying form would be exactly the same, phonologically, as Pauls (‘multiple people named Paul’). (This example only works if your plural suffix post-sibilant allomorph is /-əz/ rather than /-ɪz/, but a similar example could probably be exhibited in the other case.) One could appeal to the differing placement of the morpheme boundary but this is unappealing.
However, the assumption that a single epenthesis rule operating between sibilants is productive across the entire English lexicon has to be given up, because ‘s < is and ‘s < has have different allomorphs after sibilants! Either they are accounted for by two different lexically-conditioned epenthesis rules (which is a very unappealing model) or the allomorphs with the vowels are actually the underlying ones, and the allomorphs without the vowels are produced by a not phonologically-conditioned but at least (sort of) morphologically-conditioned deletion rule that elides fully reduced unstressed vowels (/ə/, /ɪ/) before word-final obstruents. This rule only applies in inflectional suffixes (e.g. lettuce and orchid are immune), and even there it does not apply unconditionally because the superlative suffix -est is immune to it. But this doesn’t bother me too much. One can argue that the superlative is kind of a marginal inflectional category, when you put it in the company of the plural, the possessive and the past tense.
A nice thing about the synchronic rule I’m proposing here is that it’s more or less exactly the same as the diachronic rule that produced the whole situation in the first place. The Old English nom./acc. pl., gen. sg., and past endings were, respectively, -as, -es, -aþ and -ede. In Middle English final schwa was elided unconditionally in absolute word-final position, while in word-final unstressed syllables where it was followed by a single obstruent it was gradually eliminated by a process of lexical diffusion from inflectional suffix to inflectional suffix, although “a full coverage of the process in ME is still outstanding” (Minkova 2013: 231). Even the superlative suffix was reduced to /-st/ by many speakers for a time, but eventually the schwa-ful form of this suffix prevailed.
I don’t see this as a coincidence. My inclination, when it comes to phonology, is to see the historical phonology as essential for understanding the present-day phonology. Synchronic phonological alternations are for the most part caused by sound changes, and trying to understand them without reference to these old sound changes is… well, you may be able to make some progress but it seems like it’d be much easier to make progress more quickly by trying to understand the things that cause them—sound changes—at the same time. This is a pretty tentative paragraph, and I’m aware I’d need a lot more elaboration to make a convincing case for this stance. But this is where my inclination is headed.
 The transcription system is the one which I prefer to use for my own accent of English.
Minkova, D. 2013. A Historical Phonology of English. Edinburgh University Press.