Nasalized vowels

The words bead and bean form a minimal pair, and any native speaker of English can tell that the final consonants in these words are responsible for the contrast. Besides this obvious fact, however, there lies another difference between these two words and that has to do with the vowel sounds. While both vowels belong to the phoneme /i/, the phonetic manifestation of the vowel of the second word, bean, is nasalized and, phonetically, an [ɪ̃]. This predictable allophonic rule of English is valid for all vowels; that is, English vowels (and diphthongs) are nasalized when they occur before a nasal consonant.