The development of phonology is a process of building up complexity. This happens at both the dimensions of the inventory and of the text; and it obeys the law of unilateral foundation of the contrasts both in selection and in combination. The primitive syllable consists of an occlusion followed by an opening, like [pa].

The foundational and decisive step in the evolution of phonology is the introduction of the second articulation. This happens when [pa] and [ta] are no longer uncontrolled variants of one sign, but their phonetic difference is controlled, so signs which only differ by one containing a [p] and the other containing a [t] (step 1 of the table) can reliably mean different things. This also implies that this syllable consists of two smaller units, viz. /p/ and /a/, both of which have the status of phonemes¹ and can, in principle, be replaced independently. This creates the categories of consonant and vowel. In the latter category, the feature of height (degree of openness) opposes [i] to [a] (step 2).

On the basis of this minimum, the phoneme system is enriched by using additional phonetic features. In the vowels, back vowels like [u] are opposed to front vowels like [i] (step 3). Then features like roundedness, degrees of height, degrees of backness can be added. As a matter of fact, the number of vowel qualities known from existent languages varies between two and 14. The vowel system may also be expanded by features like nasalization and glottalization, which have the potential of doubling the inventory. In the consonants, more points of articulation like velar – for [k] – are added (step 4). Voiced consonants contrast with voiceless ones (step 5), fricatives contrast with stops (step 6), consonants may be nasal (step 7). More such features exist, and some of them have the potential to double the inventory. The number of consonants known from existent languages varies between six and 122.

On the textual dimension, sequential phonological structure can become more complex. Primitive words are monosyllabic. An initial step in the development of complexity consists in the combination of two or even more syllables into one word, producing forms like [tapi]. Then syllable structure can become more complex. While quite a few languages abide by the optimal syllable consisting of a consonant followed by a vowel, which provides the basis of the entire development (s. next table), there are several options for expansion:

A consonantal coda may be added to the syllable, as in step 1 of the table. The onset may consist of a consonant cluster, as in step 2. The peak of the syllable may be a diphthong (step 3). The syllable may be further complicated by adding tone features (step 4). Again, the complex structures presuppose the simpler ones.

¹ In linguistics, a phoneme is not just a simple speech sound – a phone –, but an abstract entity comprising a set of phones which are variants of each other.