4.4 Natural Classes

It’s possible to describe an individual speech segment in terms of its phonetic features. It’s also possible to group sounds that share features into natural classes. Natural classes of sounds tend to behave similarly because they have features in common. We can distinguish obstruents, sonorants, glides, and vowels using the major class features, and distinguish in more detail within these major classes using features for place and manner of articulation.

Check Yourself

1. In the set of segments listed below, which segment must be excluded to make the remaining segments constitute a natural class?

  • [i]
  • [e]
  • [æ]
  • [o]
  • [u]

2. In the following set of segments, which segment must be excluded to make the remaining segments constitute a natural class?

  • [p].
  • [f].
  • [t].
  • [k]

3. This set of segments constitutes a natural class: [i ɛ æ]. Which segment could you add to the set while still preserving the natural class?

  • [ɪ].
  • [ɔ].
  • [ə].

Video Script

We saw in the last unit that a natural class is a group of speech segments that have some features in common. Because they share features, they also tend to behave similarly to each other in the grammar of a language. We also learned how to use the notation of a feature matrix to describe the features that the members of a natural class have in common. Remember that the more features we list, the smaller the natural class gets, and the fewer features we list, the larger the class we’re describing.

The two largest natural classes can be described with just one feature! All segments that have an obstruction in the vocal tract are [+consonant], and the vowels, which have no obstruction in the vocal tract, are [-consonant]. Notice that the feature consonant is binary,  meaning it has exactly two values: plus or minus. This means that we don’t need a separate feature to label the sounds that are vowels: using the binary feature notation we can just say that consonants are [+consonant] and all vowels are [-consonant].

Now, why did I say that most consonants are [+consonant]? Could there be consonants that aren’t totally consonants? Think back to when we first started learning to categorize sounds.  Remember we said the glides /j/ and /w/ have very little obstruction in the vocal tract. So even though /j/ and /w/ often behave like consonants, they get labelled as [-consonant] in the feature system. This is how we indicate that also they share some properties with vowels, namely, the properties of having the vocal tract unobstructed.

The next feature that groups segments into the major classes has to do with sonority, or acoustic energy. Because of the relatively open vocal tract, all vowels are [+sonorant]. But not all consonants are [-sonorant]. Approximants also have a relatively open vocal tract, so all glides and liquids are also [+sonorant]. And when air circulates through the nasal cavity, that also creates a lot of acoustic energy so all the nasal consonants are also [+sonorant]. All other sounds, that is, stops, fricatives, affricates, and so on, are minus sonorant because they have lower sonority.  Informally, we call the non-sonorant class of sounds obstruents, but we don’t need a separate feature label for obstruents because of how the binary feature system works: we just identify them as [-sonorant].

The third major feature groups sounds according to whether they can be the nucleus of a syllable or not. There’s a lot of overlap between sounds that are [+sonorant] and [+syllabic], but the two classes aren’t totally identical. I’m sure you can already guess that all vowels have the feature [+syllabic]. By default, all consonants are [-syllabic], but remember that some consonants can sometimes serve as the nucleus of a syllable — do you remember which ones? The liquids and nasals become [+syllabic] only when they’re the nucleus of a syllable.

These three features are called the major class features because they allow us to group segments into these broad categories. At one end of the spectrum, we have the vowels, with no obstruction in the vocal tract, so they’re sonorant, and they can serve as the nucleus of a syllable. At the other end, we have the obstruents: these are the consonants that have low sonority and can’t be the nucleus of a syllable. And then in between, we have the sonorant consonants, which are usually [-syllabic] but can become [+syllabic], so if we don’t list that feature, we indicate that it can be either plus or minus. And then there are the glides, which are almost like vowels except they’re [-syllabic]. There are a few details to notice here: the nasals and liquids are consonants that are sonorant, and that can become syllabic. The glides are sonorants, but they don’t serve as the nucleus of a syllable, and they don’t even count as consonants!

The next set of features we should consider are those that describe manners of articulation. The continuant feature has to do with whether air is allowed to flow in the oral cavity (that is, in the mouth). So the main job the continuant feature does is to distinguish between the stops, which are [-continuant] because airflow is blocked, and all other sounds. Notice that the nasals count as [-continuant] along with the plosive stops because they involve a complete closure in the mouth.

Speaking of nasals, the feature [+nasal] labels nasal sounds.  /m n ŋ/ are obviously [+nasal], and in English, all other sounds are [-nasal] by default.  But sometimes assimilation leads a vowel to take on a nasal feature.  In the word pants, for example, the [æ] vowel becomes nasalized in anticipation of the following nasal sound, which we would label as a change from [-nasal] to [+nasal].

The voice feature is pretty obvious: if the vocal folds are vibrating, then a segment is [+voice]. So all the voiced consonants are [+voice] and the voiceless consonants are [-voice]. In English, the vocal folds vibrate for vowels, so the vowels are also [+voice].

Now let’s look at the features that describe which articulators are active in producing a sound. Notice that the notation for the place features is different: these ones are not binary features so there’s no plus or minus, and they’re written in all capital letters to distinguish them from the binary features. The idea is that there’s no known language that makes meaningful phonological distinctions on the basis of what a sound’s place of articulation is NOT, so we don’t need a minus-value for these features. The feature [LABIAL] identifies any sound that involves the lips. [CORONAL] classifies sounds that are produced using the tip of the tongue, and [DORSAL] classifies sounds that are produced with the body and back of the tongue.

Let’s start by looking at labial sounds.  If a sound doesn’t involve the lips at all, then we don’t even list [LABIAL] in its feature matrix.  But if it does involve the lips, then we specify whether the lips are [+round], like the rounded vowels and the glide [w], or [-round], like for the bilabial and labio-dental consonants.

We label sounds that are made with the tip of the tongue with the feature [CORONAL], and then we can classify them further according to where the tip is.  The word anterior is just a fancy word for “front”.  The dental and alveolar sounds are made with the tip of the tongue towards the front of the mouth and are [+anterior]. Post-alveolar sounds have the tip of the tongue pointing farther back so they’re [-anterior]. There’s another distinction for the coronal sounds. We use the label [+strident] for the sounds that are acoustically noisy and sound like hissing, that is, [s z ʃ ʒ tʃ dʒ]. The other coronals are [-strident].  You’ll see why this distinction matters in one of the exercises at the end of this chapter.

Sounds that are made not with the tip of the tongue but with the tongue body have the feature [DORSAL].  Only a few consonants in English have the feature [DORSAL]: the velar sounds [k ɡ ŋ] and the glides [j w]. And because the body of the tongue is the primary articulator that we use to make different vowel sounds, all vowels have the feature [DORSAL].

Once we’ve identified a sound as a dorsal sound, then we use binary features to specify the position of the tongue body. We specify vowel height with the features [±high] and [±low]. All high sounds are [-low], and all low sounds are [-high], but there are also vowels that are both [-low] and [-high]: the mid vowels!  We identify the front or back position of the tongue with [±back], but we don’t need to include a [±front] feature because these central vowels pattern with the natural class of back vowels.

There’s one other feature we need for categorizing the vowels of  English, and that’s the tense/lax distinction. We label the tense vowels as [+tense] and the lax vowels as [-tense].

There’s a full chart of features for the segments of English presented below, but don’t be intimidated by it. A feature matrix is just a more organized way of presenting the information that you already learned about phonetic segments. If you start to think of segments in terms of natural classes and what features the natural class has, you’ll start to get the hang of it.

p b t d k ɡ f v s z θ ð ʃ ʒ m n ŋ l ɹ j w
Major Class Features
[consonant] + + + + + + + + + + + + + + + + + + +
[sonorant] + + + + + + +
Manner Features
[nasal] + + +
[continuant] + + + + + + + + + + + +
[lateral] +
[voice] + + + + + + + + + + + + + +
Place Features
[round] +
[anterior] + + + + + + + + +
[strident] + + + +
[high] + + + + +
[back] + + + +


i  ɪ e  ɛ  æ u  ʊ o ʌ  ɔ ɑ
Major Class Features
[sonorant] + + + + + + + + + + +
[syllabic] + + + + + + + + + + +
Manner Features
[continuant] + + + + + + + + + + +
[voice] + + + + + + + + + + +
Place Features
[round] + + + +
[high] + +  – + +  –
[low]  +  – +
[back]  –  –  –  – +  +  +  +  +  +
[tense]  +  –  +  – +  –  +  –  –  +



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4.4 Natural Classes by Catherine Anderson is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

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