(n.) a linguistic phenomenon in which “y” sounds are replaced with “sh” or “zh” sounds
Have you ever wondered why ‘got you!’ becomes gotcha!, despite there being no “tch” sound in the original phrase? And what about forms like whatcha, betcha, dontcha or wontcha? In isolation, contractions like these arguably don’t seem to make much sense given the raw materials they’re built from—and for that, you can thank a linguistic curiosity called yod-coalescence.
Used in various branches of language study, yod—a word derived from the Hebrew letter yodh, ( י )—is just another name for the “y” sound, as in you, young or yes.
Technically, this sound is called a palatal approximant, as it involves the soft palate in the centre of the roof of the mouth (hence palatal), and the tongue rising up to meet the palate but not quite rising far enough to fully disrupt the airflow through the mouth (a characteristic of all speech sounds known as approximants). In the phonetic alphabet, this sound is somewhat confusingly represented by a lowercase J, /j/ (while /y/ is used to represent the “oo” vowel found in words like the French tu and du). Referring to this /j/ sound as yod ultimately works as a reminder that, despite appearances, it is indeed a “y” sound, not a “j” sound.
For now, let’s stick with convention, and say that the sound we’re actually dealing with here is /j/.
In phonological terms, this /j/ sound is classed as a semivowel, as it effectively exhibits both vowel-like and consonant-like qualities. In phonology, the definition of a vowel sound is one in which airflow through the mouth is never blocked or disrupted, as it is with consonant sounds: you can try this yourself by thinking about how “ee” and “ooh” sounds are produced, compared with the closures and blockages your mouth makes to form sounds like “b”, “f”, and “k”. When it is formed in the mouth, /j/ is produced in much the same way as a vowel sound is, with a near constant, unoccluded airflow through. But /j/ isn’t a vowel, as unlike other vowel sounds it cannot form the middle part or ‘nucleus’ of a syllable. This curious hybridized makeup ultimately makes /j/ particularly susceptible to sound change and alteration in spoken language—especially when it occurs in clusters of other sounds demanding different forms of articulation.
Yod-rhoticization, for instance, is a feature of some American accents in which a “r” sound is introduced into words where a “y” sound would normally be expected. In some accents, a word like music becomes more like “mrusic”, while cute effectively becomes “crute”. Yod-dropping is another accent feature, in which the /j/ sound is omitted entirely from some words wherein it is retained in other accents. It’s this, for instance, that explains why words like juice and shoe are pronounced “j-YOO-s” and “sh-YOO” by some people, and simply “joos” or “shoo” by others.
Yod-coalescence is another of these yod-changing processes. It occurs in four distinct scenarios in English words where the /j/ sound is immediately preceded by a consonant to which it becomes fused—or ‘coalesced’—to produce a completely different sound combining elements of them both.
First of all, in words in which “y” follows a “d” sound, the resulting “dy” combination sometimes morphs into a “dzh” sound, like that at the end of the word edge (a sound technically known as the palato-alveolar affricate again, /dʒ/). Secondly, when “y” follows a “t” sound, the resulting combination is often more like the “tch” sound at the end of watch (the unvoiced post-alveolar affricate, /tʃ/). Thirdly, when it follows a “s” sound, “y” creates a “sh” sound (called the unvoiced post-alveolar fricative, /ʃ/). And fourthly, when it follows a “z” sound, “y” combines with it to form a “zh” sound (called the voiced post-alveolar fricative, /dʒ/), like the sound in the middle of words like vision or incision. In practice, these four scenarios are the reason why (1) a word like soldier is often pronounced “sol-dzhuh”, and not “sol-dyuh”; (2) why a word like stature is often pronounced “statch-uh” not “stat(ch)-yuh”; (3) why a word like fissure is often pronounced “fish-uh”, not “fis-yuh”; and (4) why a word like measure is pronounced “mezh-uh” by some people, and “mez-yur” by others.
Unlike other phonological changes involving the yod sound (also known by the somewhat less glamorous name of Y-cluster reductions), yod-coalescence can occur across syllable boundaries—meaning that, for instance, the “d” and “y” sounds affected don’t even have to belong to the same word in order for the change to take place. It is this more expansive form of yod-coalescence that occurs in all those examples like gotcha, whatcha and dontcha, in which the neighbouring “t” and “y” sounds (got you, what [are] you, don’t you) are split between two different words.