In formulating hypotheses about language it has been found that languages have intricate structure at a number of different levels. Phonetics studies the sounds of speech and how they are produced and perceived. Phonology studies the principles by which the sounds of a language are organized into a system and combined into syllables and larger units. Morphology studies the principles by which smaller units of meaning are combined into words. Syntax is the study of the principles by which words are combined into larger units such as phrases and sentences. Semantics studies meaningthe meanings of words and the ways the meanings of words are related to the meanings of larger units such as the phrase, the sentence, and the discourse. Linguists attempt to discover to what extent the principles at each level vary across languages, and to what extent they are universal. These principles apply to both spoken languages and signed languages.