 Illusion is the phonological process in which a phoneme that is present in the citation form of a word is deleted under the influence of the context. In most cases, illusion occurs flanking, where the position of an elided phoneme is at the edge of a word or a syllable in its citation form. Flanking illusion primarily affects alveolar plosive consonants as in last time, where the final alveolar plosive of last disappears under the influence of the next word, or in old Ted, where the final D in old is elided. Sometimes, illusion comes together with assimilation as in can't be, where due to the elision of the final consonant in can't, the now final alveolar nasal becomes bilabial under the regressive influence of B. Word internal illusion affects both vowels and consonants, whereas the elision of vowels is a result of the rhythmic structure of English as in frightening or history, where unstressed vowels may disappear. Consonantal word internal illusion follows the central principle where the middle consonant of three adjacent consonants is dropped, as in windscreen. However, in all these cases of present-day English, elision is mainly optional and typically occurs in rapid conversation.