Some phonological rules are neither assimilation nor dissimilation rules. The aspiration rule in English, which aspirates voiceless stops at the beginning of a syllable, simply adds a non-distinctive feature. As we did in the nasalisation rule earlier, we can use the symbol $ to represent a syllable boundary. Generally, aspiration occur only if the following vowel is stressed. The /p/ in pit and repeat is aspirated but the /p/ in in $ spect or com $ pass is usually unaspirated (although if aspirated it will not change meaning since aspiration is non-phonemic). Using the feature [+ stress] to indicate a stressed syllable and V` to symbolise stressed vowels, the aspiration rule may be stated: