The soprano is the highest female voice in four-part harmony. Written music places her notes in the treble clef. She has a typical range of middle C to high A and often carries the melody in a song.
Alto is the second-highest voice in a four-part singing group and usually belongs to a female. Although music theory has an alto clef, an arranger usually writes the alto singing part in treble clef. On average, she sings between the G below middle C and the F two octaves above.
The tenor, usually male, has the second-lowest singing voice in a quartet. A tenor part usually appears in the treble clef or occasionally bass clef. In the treble clef, the tenor sings an octave below the note indicated. The tenor range begins with C below middle C and typically reaches the A above middle C.
The bass is the lowest voice in a four-part singing group and sings music written in the bass clef. His average range is the second F below middle C to the E above middle C. More than any other part, the bass often sings an accompanying part, rather than a melody, often a "continuo" part that establishes the root note of the harmony.
An all-female singing quartet may comprise just sopranos or a mixture of sopranos and altos. A male group such as a barbershop quartet may have a combination such as three tenors and one bass, or two of each.