MATRES LECTIONIS
Matres lectionis (from Latin "mothers of reading", singular form: mater lectionis,
from Hebrew: אֵם קְרִיאָה 'em kri'a) are consonants that are used to indicate a vowel, primarily in the writing of Semitic languages such as Arabic, Hebrew and Syriac.
The letters that do this in Hebrew are aleph א, he ה, vav ו and yod י,
and in Arabic, the matres lectionis (though they are much less often referred to thus) are ʾalif ا, wāw و and yāʾ ي.
The 'yod and waw in particular are more often vowels than they are consonants.
The original value of the matres lectionis corresponds closely to what is called in modern linguistics glides or semivowels.[1]
Historically, the practice of using matres lectionis seems to have originated when /aj/ and /aw/ diphthongs, written with the yod י and the waw ו consonant letters respectively, monophthongized to simple long vowels /eː/ and /oː/. This epiphenomenal association between consonant letters and vowel sounds was then seized upon and used in words without historic diphthongs.
In general terms, it is observable that early Phoenician texts have very few matres lectionis, and that during most of the 1st millennium BCE, Hebrew and Aramaic were quicker to develop matres lectionis than Phoenician.
However, in its latest period of development in North Africa (referred to as "Punic"), Phoenician developed a very full use of matres lectionis, including the use of the letter ayin ע, also used for this purpose much later in Yiddish orthography.