What are natural classes in phonology? Can phonological features make a set of segments a natural class?

For example, is there any way to make a natural class out of the set:

[k, x, q,χ]?

What are the phonological features that can make the above set a natural class?

2 Answers 2


A "natural class" in a lot of spaces refers to "a group of things with a simple connection between all of them, that you can use to determine what is or isn't in the set." In phonology, it is frequently used in autosegmental theories - or theories which see each phoneme as actually a bundle of features. For example, the set

[k, x, q, χ]

Contains only elements which are [+dorsal -voice +obstruent]

And, is in fact, pretty much the set of "segments" (the word they use instead of "phones" or "sounds") which have those three features, so form a natural class.

  • isn't this too general? because the features [+dorsal -voice +obstruent] include so many other segments?
    – User384789
    Feb 10, 2020 at 0:18
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    It doesn't account for things like coärticulation, vocalic uses of those phonemes, etc., but having every single feature is confusing and not helpful if you aren't working with a specific language, because every language cares about different contrasts Feb 10, 2020 at 0:23
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    @User384789 For example, in a particular language, /k/ and /k:/ might contrast, in which case these four are also [-long]. But in a language without length contrasts, [±long] isn't a thing, so it doesn't make sense to specify it.
    – Draconis
    Feb 10, 2020 at 0:26
  • Don't try to do minimal stuff at first. Use all the features you need, because features are imaginary and part of the theory. You can refine the theory after you make sure it covers all the data. That's much more important than being elegant; this is science, not mathematics.
    – jlawler
    Feb 10, 2020 at 2:51

The term "natural class" is traditionally used to refer to a collection (class) of sounds that is "natural". When paired with a theory of phonetically-based phonological features, is has been assumed that natural classes are all of the possible conjunctions of features in the right theory of features. For example in the SPE theory of features, the sets [p,t,k], [p,b,m] or [f s θ x v x ð] are natural classes that can be expressed by the feature expressions [-son,-cont,-voice], [+ant,-cor,-cont] and [+cont,-son] respectively. Typically such classes are of interest because phonological rules operate on classes of sounds, in the context of other classes of sounds, and create output classes with some defining property in common. [k,x,q,χ] can be framed as a class in many ways, depending on the theory of features that you assume. In the SPE system, these segments are [+back,-voice] and in certain feature geometric approaches that are Dorsal and voiceless (or just Dorsal, not specified for voicing).

I should point out that to specify exactly [k,x,q,χ] excluding all other possible woulds, you have to specify a lot of other things – [-spread glottis, -constricted glottis, -round, -delayed release, -coronal...] – the exercise of specifying classes of segments using features only makes send in the context of a specific language which has a fixed set of segments to be compared to.

  • This is not quite right. A natural class for some language includes all the phones of the phones of that language that share a phonetic character. So, for instance, if your example of [p,t,k] characterized as "[-son,-cont,-voice]" is a class of phonemes in a language with distinctive rounding of consonants, the rounded versions of [p,t,k] should have included also the rounded versions of [p,t,k], or else the characterizing set of features should have included [-rnd].
    – Greg Lee
    Feb 10, 2020 at 1:42

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