[*] 5. The vowels i and u serve as consonants when pronounced rapidly before a vowel so as to stand in the same syllable.1 Consonant i has the sound of English consonant y; consonant u ( v ) that of English consonant w. Consonant i and u ( v ) are sometimes called Semivowels.
[*] Note 1.--The Latin alphabet did not distinguish between the vowel and consonant sounds of i and u, but used each letter (I and V) with a double value. In modern books i and u are often used for the vowel sounds, j and v for the consonant sounds; but in printing in capitals J and U are avoided:—IVLIVS ( Iūlius ). The characters J and U are only slight modifications of the characters I and V. The ordinary English sounds of j and v did not exist in classical Latin, but consonant u perhaps approached English v in the pronunciation of some persons.
[*] Note 2.--In the combinations qu, gu, and sometimes su , u seems to be the consonant ( w ). Thus, aqua , anguis , cōnsuētus (compare English quart, anguish, suave). In these combinations, however, u is reckoned neither as a vowel nor as a consonant.2