ZAYIN (ז). This seventh letter of the Heb. alphabet was originally written with two horizontal strokes bisected by a vertical line dropping well below the lower bar. By the 11th cent. b.c. the vertical simply connected the two horizontals, much like a capital H lying on its side; later on the horizontals shortened, making it more like a capital “I.” With the advent of “square Hebrew” the bottom horizontal was dropped completely and the top bar was tipped slightly SE. Numerically zayin served as the numeral seven. It was apparently pronounced exactly as “z” in Eng. In some words zayin replaced the pre-Hebraic phoneme dh (pronounced like th in “this”), both in Heb. and in Old Aram.
International Standard Bible Encyclopedia (1915)
"z": The 7th letter of the Hebrew alphabet; transliterated in this Encyclopedia as "z". It came also to be used for the number 7. For name, etc., see Alphabet.