The term 'Half of a 60's Quartet' refers to the popular singing group The Mamas and the PAPAS, who were active in the 1960s. The crossword solutions for this term are 'PAPAS' (5 letters) and 'Mamas' (5 letters). The Mamas and the PAPAS were an American folk rock vocal group that rose to fame in the mid-1960s with their signature sound of close harmony singing. They had several hit singles, including 'California Dreamin'', 'Monday, Monday', and 'Dedicated to the One I Love'.