Most commonly, yod indicates i or e, while waw indicates o or u. Aleph was not systematically developed as a mater lectionis in Hebrew (unlike in Aramaic and Arabic), but it is occasionally used to indicate an a vowel. (However, a silent , indicating an original glottal stop consonant sound that has become silent in Hebrew pronunciation, can occur after almost any vowel.) At the end of a word, he can also be used to indicate that a vowel a or e should be pronounced.
