Phonological Universals

What is a phonological universal?
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Definition
A phonological universal is a common tendency found in the phonological systems of many languages.

Examples: Vowel systems

Here are some phonological universals concerning vowel systems:

Symmetry
Vowel systems tend to be symmetrical.
The minimal vowel system includes /i a u/. All known languages are said to have these three vowels, or slight variations of them.

Rounding
Back vowels tend to be rounded.
Front vowels tend to be unrounded.

A functional explanation for these vowel universals is that, in its vowel system, a language is likely to use those vowels that are the most perceptually different from one another. This makes it easier for the listener to distinguish between the vowels in the system.

Examples: Consonant systems

Here are some phonological universals concerning consonant systems:

Symmetry
Consonant systems tend to be symmetrical.

 A correlation between point of articulation and voicing of obstruents shows up when there are asymmetries in a consonant inventory.
A language is less likely to have voiceless labial obstruents than any other voiceless obstruents.
A language is less likely to have voiced velar obstruents than any other voiced obstruents.

 All languages are expected to have at least the following consonant phonemes:
Voiceless plosives (stops)
Nasals
A grooved fricative (for example, /s/)
A laryngeal glide (usually /h/)

Voicing
Most obstruents are voiceless
Most sonorants are voiced.

Example: Nasality

Here are some phonological universals concerning nasality, a process that commonly interacts with both consonants and vowels:

Consonants
Nearly all languages have nasal consonants.

Vowels
Nasal vowels are usually the exact counterparts of the oral vowels.
If a language has fewer nasal vowels than oral, it is usually the mid nasal vowels that are missing.

In the “Experimental Approaches to Phonology”
Phonological universals are understood here as resulting from production and perceptual constraints shaping spoken language. Besides being universal, these forces influence language in a probabilistic way. Broadly speaking, production constraints stem from neurological, anatomical, physiological, and aerodynamic conditions, as well from the mapping between vocal tract shape and the resulting acoustic signal, while perceptual constraints derive from the peripheral and central auditory transforms of speech, lateral inhibition, masking, critical bands, short-term memory, and the way in which sounds are stored and retrieved.