The classic Latin alphabet had 23 letters. The letters J, U and W have been added during the middle ages. They were originally just variants of the Latin letters I and V.

The origin of "u" is as the lowercase form of "V". Latin had used "V" both for the vowel form of "u" and for the consonant form of "u", which is written in English as "w". The Latin consonant "u" has become fricative in the Romance languages, i.e. the sound written in English as "v".

Due to this change in pronunciation, for the Romance languages 2 new letters have been created, uppercase "U" and lowercase "v", pairing them with the older "V" and "u" and giving them the current pronunciation. Most other European languages have then taken this updated Latin alphabet. A similar history was for the splitting of Latin "I" into "I" and "J".

English is one of the few Indo-European languages that has preserved the original consonant "u" pronunciation in many words, instead of changing to a fricative "v" pronunciation. Because of this, "W" has been originally created for English to write this pronunciation, for which neither "u" nor "v" was appropriate. Later "W" has been taken also by other languages, where it has received different pronunciations.