There are 35 Khmer consonant symbols, although modern Khmer only uses 33, two having become obsolete. Each consonant has an inherent vowel: "â" or "ô" ; equivalently, each consonant is said to belong to the "a"-series or "o"-series. A consonant's series determines the pronunciation of the dependent vowel symbols which may be attached to it, and in some positions the sound of the inherent vowel is itself pronounced. The two series originally represented voiceless and voiced consonants respectively (and are still referred to as such in Khmer); sound changes during the Middle Khmer period affected vowels following voiceless consonants, and these changes were preserved even though the distinctive voicing was lost (see phonation in Khmer).