Vowels are fundamentally different from consonants in several ways that affect how they can change. First, all vowels are voiced, so there cannot be a change in voicing. Second, all vowels are said in the same manner, so there cannot be changes in manner either. Finally, vowels are not produced by placing the tongue in contact with discrete speech organs, so there can be no movement from one part to another either. The features of vowels that can change are frontness, height, roundedness, and tenseness. The causes of this change can be neighboring consonants, although that is rare. More often, the change is a “long-distance” change in that it is produced by assimilating with vowels in a preceding or following syllable (since vowels form the nucleus of a syllable, other vowels are most often in other syllables). This type of change, where one vowel is influenced by another, is called metaphony.

One type of metaphony vowels undergo is mutation, where a vowel in one syllable anticipates a vowel in a following syllable and changes to become more like it. For example, in a two-syllable word with the pattern CV₁CV₂C, where C = Consonant and V = Vowel. In mutation V₁ changes to become more like V₂, because it is easier for the tongue to anticipate the shape it will need to be in for the following vowel. In a word in which V₁ is a low vowel and V₂ is a high vowel, V₁ could be “raised” to a high vowel to anticipate the height of V₂ in the following syllable.

Perhaps the most common example of mutation in English occurred just before the Old English period, when V₂ was an [i], a high front vowel. This caused the V₁ vowels to mutate towards the front of the mouth. We call this change either front mutation, or i-mutation, or in German, Umlaut, and it has had major effects throughout English. Compare the list of words with back vowels with related words with fronted vowels in the second:

• blood – bleed
• food – feed
• mouse – mice
• man – men
• drank – drench
• strong – strength
• full – fill

In all these examples the words on the left hand are the original words. In the period just before Old English, new words were made out of them by adding suffixes. The suffix in each case had the vowel [i], and this pulled the vowel in the original word to the front.

Chain shifts

Vowels can also move in what are called chain shifts. This is not a form of assimilation or metaphony. Rather, it is a process of movement that starts with the raising or lowering of one vowel, which results in a chain of all other vowels moving to fill in the resulting gaps. The most famous one in English is the Great Vowel Shift of the 15th through the 18th centuries, which we will cover in detail later, but other chain shifts are currently going on in regional dialects of American English, such as the Northern California vowel shift. Another one, the Northern Cities Vowel Shift, explains why vowels of speakers from places like Chicago or Detroit sound so distinctive.

Reduction

Reduction is a change in the tenseness of vowel sounds, resulting in less tenseness. Vowels that are reduced change in a progression from tense to lax to schwa to zero or null, i.e., they are lost entirely. This is the standard progression in all English vowels in unaccented syllables, although not all vowels have or will undergo each stage of this progression, and it has had more of an impact on English grammar than anything else.

An example of reduction is the vowel in the word “man,” [mæn], a low, front vowel. When the word stands alone and is stressed, the vowel remains in position. When the word is joined in a compound, as in “chairman,” it is no longer stressed and therefore is reduced. It may be pronounced as a schwa (i.e., moved from the front to the central, that is, neutral, position) [tʃɛrmən], or even reduced to zero: [tʃɛrmn̩].

When reduction of a vowel is complete (i.e., the vowel is lost completely), then it is called elision: the loss of a sound because it is unstressed (elision can be used of both consonants and vowels). Elision is widespread in two of our major grammatical categories: the s-plural and the ed-past tense. The plural ending used to be a full syllable, as it remains in some words (“dishes”), but in most words the –e- has reduced to zero, that is, it has been elided. The same reduction has happened in the past tense –ed, which usually takes the form of either a [t] or a [d] on the ends of verbs.

Conditioned vs Unconditioned changes

Most of the sound changes discussed for consonants and vowels are conditioned changes, meaning they occur only when the sound occurs in a specific condition or environment. Thus, assimilation happens because of a certain condition of the word causing a specific consonant to changes its manner, place or voice. So, not every [n] will become [ŋ], but only the ones that occur before a velar sound, as in “pancake” and “incredible.” In fact, any change made by the principle of least effort is a conditioned change. The result is that different people in different regions make different choices about what sounds require least effort, and so different regions end up speaking differently depending on what changes are “acceptable” within their linguistic community. Only some dialects may undergo palatalization, where others may “decide” that assimilation of place creates too much difficulty in comprehension and so avoid it.

Other changes, however, like the change shifts in Northern California or the Northern Cities, or the cot-caught merger, are unconditioned. In unconditioned changes, all instances of a particular phoneme or allophone change, regardless of the phonetic condition it occurs in. Thus, for some speakers of American English, all instances of [ɔ] merged with [ɑ] so that there is now for them no distinction in pronunciation between the words “caught” and “cot” and all other words with those same vowels – they are both pronounced with [ɑ]. This merger did not happen because of a preceding [t] or a following back vowel or anything else in the word, nor is it a change that happened to make the word easier to say, nor does it affect only the word “caught.” There is no specific cause or condition that can be identified for why the merger took place.

As with conditioned changes, unconditioned changes happen differently in different dialects. The Northern Cities shift is occurring in only certain cities in the northeast and northern midwest of the U.S. The caught-cot merger only happened in parts of the U.S. Eventually, so many little changes, conditioned and unconditioned, happen in different dialects that it becomes impossible to understand what a person speaking that dialect is saying. Their phonemes have shifted to a different set than the one used by other speakers. That is one way language changes over time and diverges into multiple languages.