In language typology, languages are classified based on various features of their sound systems, including the number of vowels, tonal patterns, and syllable structure.

Languages with a relatively small number of vowels are often referred to as vowel-poor languages, while languages with a relatively large number of vowels are referred to as vowel-rich languages.

In tonal languages, each syllable has a distinct tone, and these tones can change the meaning of words or mark grammatical categories.

Languages with a complex syllable structure are often referred to as syllable-rich languages, while languages with a simple syllable structure are referred to as syllable-poor languages.

In mora-based languages, each syllable has a distinct mora, and the number of moras in a word determines its length.

Deletion is not a common type of phonological process, as it involves the complete removal of a sound from a word.

Assimilation is the process by which two adjacent sounds become more similar in articulation.

Dissimilation is the process by which two adjacent sounds become less similar in articulation.

Metathesis is the process by which two adjacent sounds switch their positions.

Obligatory rules, optional rules, and variable rules are all common types of phonological rules.

An obligatory phonological rule is one that applies to all words in a language.

An optional phonological rule is one that applies to some words in a language.

A variable phonological rule is one that depends on the context in which a sound occurs.

Phonological rules are used to describe the sound patterns of a language, predict the pronunciation of words, and explain the historical development of a language.

The study of sound systems in language is known as phonology.