IPA Zounds models language sound change by applying a given set of sound change rules to a given lexicon. It has a built-in model of the IPA, allowing users to write input words in IPA characters, and rules using those characters and/or the distinctive features of the model. So, for example, a rule to lengthen any vowel before a final voiced postalveolar fricative would be written as: [+syllabic]/[+long]/_ʒ#.