In our last unit, we learned about the notion of a phoneme. Remember that a phoneme is something that exists in your mind: it’s like a shopping bag in which your mind stores memories of examples of phonetically similar sounds that are all members of one category. Not all the sounds that you store in one phoneme category have to be identical; in fact, your mental category has room for a lot of variation. Any variants that are not contrastive, that don’t lead to a meaning change, are members of that same phoneme category and are called allophones.
We’ve already seen some examples of allophones of English phonemes as we’ve been learning to transcribe sounds. We know that the alveolar lateral approximant [l] has a voiceless variant [l̥] and a syllabic variant [l̩], but our minds categorize all of them as members of the same phoneme. This shopping-bag metaphor is going to get a little unwieldy, so let’s look at another notation that we can use to represent this phoneme category.
We say that /l/ is the label for the phoneme category itself, it’s the most general form of the phoneme. Notice that instead of using square brackets, for the symbol that represents the whole category we use slashes. In any given word, the phoneme /l/ might get spoken as any one of its allophones, each of which gets represented in square brackets. But where does each allophone appear? Which allophones do we use in which words? One of the big things that phonology is concerned with is the distribution of allophones: that is, what phonetic environments each allophone appears in. The distribution of allophones is a key part of the mental grammar of each language — it’s something that all speakers know unconsciously.
Some allophones appear in free variation, which means that it’s pretty much random which variant appears in any environment. But most allophones are entirely predictable: linguists say that allophonic variation is phonetically conditioned because it depends on what other sounds are nearby within the word.
Let’s start by looking at free variation because it’s the simpler case. Take our phoneme /l/, as in the words lucky and lunch. Most of the time you pronounce these words with a plain old ordinary voiced alveolar lateral approximant. But sometimes you might be speaking extra clearly — maybe you’re trying to talk to a relative who’s hard of hearing, or maybe you’re concentrating on teaching some speech sounds to a language learner. So instead of making the /l/ sound at the alveolar ridge, you stick your tongue right out between your teeth and say lucky or lunch. Now you’re making a dental [l̪], not an alveolar [l], but it’s still a member of the phoneme category for /l/ — it doesn’t change the meaning of the word so this phonetic difference is not contrastive. It’s just free variation within the category.
But most allophonic variation is predictable: different allophones show up in different environments. Let’s look at a few words. If we look at this set of words: plow, clap, clear, play, we can see that whenever /l/ follows a [p] or [k], it is devoiced. But now look at this other set of words (blue, gleam, leaf, fall, silly), when /l/ appears in any other environment, like following a voiced stop, or at the beginning of a word, at the end of a word, or in the middle of a word, it’s the ordinary [l]. If we looked at a whole lot more words and recorded a lot of English speakers, we’d find that whenever /l/ is in a consonant cluster following a voiceless aspirated stop, it also becomes voiceless, but when /l/ is in other environments, it stays voiced. We never find voiceless [l̥] in other environments, and we almost never find voiced [l] following a voiceless stop. That pattern is called complementary distribution.
That’s an important phrase, and it’s going to come up a lot in the next few units. It means that there’s no overlap in where we find the allophones: We see voiceless [l̥] following voiceless stops, but never anywhere else, and we never see voiced [l] in that environment. Likewise, we see voiced [l] in lots of different environments, but we never see voiceless [l̥] in any of those places. When we see complementary distribution, that’s good evidence that the two segments we’re considering are allophones of one phoneme. Can you think of any other examples of English phonetic segments that are in complementary distribution? Think about what happens when you’re transcribing voiceless stops.
So let’s sum up. If we have two phonetic segments that are related but different from each other, and we find some minimal pairs to show that this phonetic difference is contrastive, then we conclude that those two segments are two different phonemes.
And if we have two phonetic segments that are related but different, and they’re not contrastive, then we look to see what the distribution of these segments is, that is, what environments we see them in. If they’re not contrastive and they’re in complementary distribution, then we conclude that they’re allophones of the same phoneme.