The system or structure of a language (langue or competence) can be described at four different levels, which form the core reas of linguistics, sometimes called microlinguistics: (1) Phonetics and phonology deal with pronunciation, or, more precisely, with peech sounds and the sound system. (2) Morphology covers the structure of words. (3) Syntax explains sentence patterns. (Morphology and syntax, often combined into morpho-syntax, have traditionally been referred to as grammar.) (4) Lexicology
and semantics describe the vocabulary, or lexicon, and explore different aspects of meaning.
The branch of linguistics that studies word structures, especially in terms of morphemes is called morphology. Traditionally, a basic distinction has been made between morphology (which is primarily concerned with the internal structures of words) and syntax (which is primarily concerned with the ways in which words are put together in sentences). A widely recognized approach divides the field into two domains: lexical or derivational orphology studies the way in which new items of vocabulary can be built up out of combinations of elements (as in the case of in-describ-able); inflectional morphology studies the ways words vary in their form in order to express a grammatical contrast (as in the case of horses, where the ending marks plurality (David Crystal, 2003).