The Romance languages underwent sound changes on the way from Vulgar Latin to becoming French, Italian, Portuguese, Spanish, etc. One of these changes that happened to all of them was the shift from the Classical Latin /w/ to /v/. As that happened, the written letter didn’t change, although its sound changed. /w/ is a bilabial approximant, articulated using both lips. /v/ is a labiodental fricative, using the lower lip and upper teeth. All that has to happen is for the teeth to get on the lips, and /w/ changes to /v/. Actually, there was an intermediate stage, [[symbol]b[/symbol]], which is a bilabial fricative, and Spanish still has this sound.
This sound shift from /w/ to /v/ is very common in many languages throughout the world; it has happened in all the Germanic languages except English, which has kept the older /w/; in Persian—the modern Iranian pronunciation has /v/ whereas Dari in Afghanistan has kept the older /w/; in Hebrew—the original /w/ has become /v/ in Ashkenazic Hebrew (probably from German influence), while Arabic has kept the older /w/. In the Indic language that came from Sanskrit, [v] and [w] share the same phoneme. It has incompletely shifted: sometimes it comes out [v], other times [w], and other times in between.
We still use “u” to spell the sound of /w/, in words like quake and guava.
English didn’t have /v/ as a phoneme until after the Norman Invasion, when Middle English began to incorporate a lot of French words. Old English did have a [v] sound, but it was an allophone of the /f/ phoneme when it came between vowels.
Originally, F, U, V, W, and Y all came from one and the same letter in the ancient Semitic alphabet. It originally looked like Y and it was pronounced [w]. The top got bent over to make F (which had the sound of [w] in Ancient Greek), and the bottom was dropped to make V. The Roman V was basically the vowel /u/, but when another vowel followed it, it had the glide sound of /w/.
The letter u was invented by Carolingian scribes in the 9th century, who developed an elegant hand called uncial with the rounded letterforms. Of course, they didn’t distinguish U from V; they used the rounded u form as lowercase for both vocalic u and consonantal v. The two were still considered one and the same letter. After printing was invented, the rounded u was used as the lowercase of both u and v, and the angular V was used as the uppercase for both.
It wasn’t until maybe the 18th century that printers decided it would be useful to have two letters there instead of one. Thus the uppercase U and the lowercase v came into being. The count of alphabet letters went up by one.
Ever wonder why W is called double U and not double V, since it’s so obviously a double V? Because V was U until only a couple centuries ago. Again, it was thought useful to develop a letter specially for the consonantal sound.
The last letter added to the alphabet, to make 26, was J. It wasn’t added until the 19th century! Before that, everyone thought of I and J as the same letter. That’s why Washington, D.C. has no J Street. The next street after Eye Street is K Street. J didn’t get its own slot in the alphabet until after the District of Columbia was platted.
