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 the J and U are avoided:---IVLIVS (Iulius). The characters J and U are only slight modifications of the characters I and V. The ordinary Englicsh 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, consuetus (compare English quart, anguish, suave). In these combinations, however, u is reckoned neither as a vowel nor as a consonant.[5][In such words it is possible that the preceding consonant was labialized and that no distinct and separate consonant u was heard.]