Brief History of W
W, the twenty-third letter of the English alphabet, was unknown in antiquity (Firmage 240). The letter W was originally descended from the Phoenician waw, which also gave birth to the F, U, V, and Y (Haley 56). The Phoenician waw was derived from a creature called Cerastes, which was a giant snake or dragon, and was depicted as an Egyptian hieroglyph to represent a consonant sound roughly equivalent to that of our F. The waw looked liked our present day Y and represented the semi-consonant sound of w as in "know" or "wing" (Haley 56). Between 800 and 900BC, the Greeks adopted the Phoenician waw, using it as the basis for the upsilon, for the [u] sound, and the digamma, for a sound equivalent to our F. The Etruscans adopted the upsilon character first, then the Romans, so the W was not a part of the early Roman alphabet. Both the Etruscans and the Romans used the upsilon to represent the semi-consonant [w] and the [u] sounds.
The Norman Conquest consolidated and expanded the use of the Roman alphabet, and soon using two U's or V's in combination (UU, VV) came to designate the [w] sound (Firmage 243). The Anglo-Saxons used a V for both the [u] and [v] sounds and wrote the V twice for the [w] sound (Haley 56). At first, due to the influence of semi-uncial and gothic scripts, the double U was the commonly used form. Then, as the classic Roman forms came into renewed favor, the double V became the accepted form (Firmage 243). The double U used first as the common form suggests why the letter is called double U instead of double V in English (some languages refer to the letter as double V).
Firmage, Richard A. The Alphabet
Abecedarium: Some Notes on Letters. Boston:
David R Godine, 1993. 240-248, 274-283.
Haley, Allan. The History, Evolution and Design of the Letters We Use Today. New
York: Watson-Guptill, 1995. 56-59, 98-99.