A term derived from TAXONOMIC studies and applied in LINGUISTICS to refer to any CLASSIFICATION of linguistic UNITS which recognizes a series of successively subordinate LEVELS. Hierarchical structure can be illustrated from any branch of linguistics, e.g. the analysis of a SENTENCE into IMMEDIATE CONSTITUENTS, or the analysis of the LEXICON into semantic FIELDS of increasing specificity (as in Roget’s Thesaurus). The relationship of inclusion which is involved can be seen in analyses of linguistic structure where DISCOURSES are said to ‘consist of’ sentences, which in turn consist of CLAUSES or PHRASES; these consist of WORDS, which in turn consist of MORPHEMES. The term has a special status in RELATIONAL GRAMMAR, as part of the phrase ‘ACCESSIBILITY hierarchy’, and has also been used with reference to CASE GRAMMAR (‘case hierarchy’). In some models of NON-LINEAR PHONOLOGY, the ‘PROSODIC hierarchy’ shows the relationship between MORA, SYLLABLE, FOOT and WORD.