In most language, the rule for combining consonants is that they should have the same voice. For example: in English, a word can never start with /zt-/ but can start with /st-/. It's not that /zt-/ can't be pronounced, it can be pronounced but needs much more effort than /st-/ because /z/ and /t/ have different voice.

Now /z/ and /d/ have the same voice but /zd-/ needs more effort. "Stop" is easy but "zdop" is difficult to pronounce.

So does /zn-/ even though they have the same voice (compare "snow" and "znow", the consonants in the former have different voice). Is there any acoustic reason for this? Why do they need more effort as compared to voiceless pair like /st-/?

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    It's just a matter of the phonotactics of English and what you're personally used to. Plenty of other languages, like Ancient Greek, have no issue with word-initial zd-; it's just about practice. – Cairnarvon Jan 14 at 10:48
  • @Cairnarvon though if a Greek zeta ζ moved from zd- to dz- to z- (this is disputed, though seems plausible to me) then there may have been issues – Henry Jan 14 at 11:19
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    @Henry I'm not aware of any evidence for zeta going from zd to dz before becoming z, though it almost certainly did go from dz to zd in the early Archaic period. I answered a relevant question on the Latin SE a while back here. – Cairnarvon Jan 14 at 11:38
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    @Cairnarvon The same is true for Russian. zd-, zn- are perfectly common combinations: здание (a building), знание (knowledge) (and many more). – tum_ Jan 14 at 12:02
  • Italian also has initial /zd/, /zb/ and /zn/ (e.g. in sdegno, sbaglio and snello respectively). I suspect these combinations are in fact not uncommon across languages, although of course one would need a study to be sure. – Denis Nardin Jan 15 at 19:23

The broadest generalization is that, whatever is systematically lacking in your language, that thing is hard to do. If your language has no [ʁ], it is hard for you to produce [ʁ]. If words do not begin with [nd, mb, ŋg], it is hard for you to product such clusters. Hence English speakers would have problems pronouncing [ŋgata], [ŋko], but most Bantu-language speakers have no problem pronouncing that. Some Bantu-language speakers have problems with [ŋko] but not [ŋgata] because in some languages, nasal plus voiceless consonant clusters don't exist, but parallel clusters with voiced stops do.

The primary reason why Modern English doesn't have initial clusters like [zb] is that Middle English didn't have then, which is because Old English didn't have then, which is because proto-Germanic didn't, as far back as the eye can see. A few people have nativized [zb] clusters, owing to a period of exposure for a couple of years to the name Zbigniew Brzezinski, but otherwise there has been no source of such clusters being added to English. North Saami, on the other hand, adopted [sp,st,sk] clusters from neighboring Germanic, because the level of contact between Saami and Germanic was much higher than the level of contact between English and Polish.

A challenging and unanswered empirical question is whether there is a specific significant disadvantage to initial [zb] clusters. If a language doesn't have consonant clusters, doesn't have /z/ or doesn't have /b/, then it is not going to have initial [zb] clusters. Once you narrow the survey down to languages with /z, b/ and with initial clusters, then you can start surveying frequency of language with no restriction against [zb] versus languages with such a restriction. Some Arabic dialects have such clusters, historically arising from /zibV.../, /zubV.../ (similar to how such clusters arose in Slavic). To even determine that there is a significant trend against [zb] (one that isn't just a function of limits on /z,b/ or clusters), you would need a broad survey of the facts, which we don't have.

I applaud the effort to consider an acoustic explanation, since people frequently attribute perceptual problems to articulatory causes: in this case, I think the explanation behind *[zb] is more physical (aerodynamic). The aerodynamic challenge of [z] is that to produce turbulence, you need rapid air flow, and to produce voicing, you need rapid airflow, so [zb] in any position is quite challenged. In initial position it is particularly challenged because airflow has to rise very quickly from zero, but then because of the stop, it will be extinguished soon. Thus initial [zb] is more susceptible to phonetic modifications, compared to [zm]. The prediction is that [zb] should be more likely to be missing compared to [zm]. Systematic statistics on this question across the world's languages are lacking.


This is just a subjective feeling that really depends on your native language or the languages you are used to speaking. Other languages are fine with this particular combination.

For example, in my native Czech it is quite fine to have non-voiced + non-voiced st- or voiced + voiced zd-. But you cannot have zt-. You can write it, but due to voicing assimilation (the first consonant must conform to the other one) it is pronounced /st-/. Similarly one cannot have sd-, it exist in written words but must be pronounced /zd-/.

zdát [ˈzdaːt]
stát [ˈstaːt]
ztenčit [ˈstɛnt͡ʃɪt]
sdružení [ˈzdruʒɛɲiː]

The situations with zn- is similar, but the nasal n does not cause the asimilation. You can have znak [ˈznak] but also snář [ˈsnaːr̝̊].

Other languages have other rules and their speakers will consider other combinations to be hard or easy to pronounce.

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