Assonance is the repetition of identical or similar phonemes in words or syllables that occur close together, either in terms of their vowel phonemes or their consonant phonemes. However, in American usage, assonance exclusively refers to this phenomenon when affecting vowels, whereas, when affecting consonants, it is generally called consonance. The two types are often combined, as between the words six and switch, which contain the same vowel and similar consonants. If there is repetition of the same vowel or some similar vowels in literary work, especially in stressed syllables, this may be termed "vowel harmony" in poetry.