Poetry is a form of literary art that uses aesthetic and often rhythmic qualities of language to evoke meanings in addition to, or in place of, literal or surface-level meanings. A particular instance of poetry is a poem and is written by a poet. Poets use a variety of techniques called poetic devices, such as assonance, alliteration, consonance, euphony and cacophony, onomatopoeia, rhythm, rhyme schemes and sound symbolism, to produce musical or other artistic effects. They also frequently organize these devices into poetic structures, which may be strict or loose, conventional or invented by the poet. Poetic structures vary dramatically by language and cultural convention, but they often rely on rhythmic metre: patterns of syllable stress or syllable weight. They may also use repeating patterns of phonemes, phoneme groups, tones, words, or entire phrases. Poetic structures may even be semantic.