The Book of the Thousand Nights and a Night… (1886)
“ But among those consonants there are three, called weak letters (Hurúf al-illah) , which have a particular organic affinity to these vowel sounds: the guttural Hamzah, which is akin to a, the palatal Yá, which is related to i, and the labial Wáw, which is homogeneous with u. Where any of the weak letters follows a vowel of its own class, either at the end of a word or being itself followed by another consonant, it draws out or lengthens the preceding vowel and is in this sense called a letter of prolongation (Harf al-Madd) . ”
