Phonology is the study of how sounds are organized and used in languages. It examines sound systems at levels below the word, including phonemes, syllables, and articulatory features. Phonology also studies equivalent systems in sign languages. It is distinguished from phonetics, which examines physical sound production and perception, while phonology describes how sounds function meaningfully in languages. The field has developed from early Sanskrit grammar to modern theories including generative phonology, natural phonology, autosegmental phonology, and optimality theory.