thirty first

X, as codified during the Y revival era (1940s-present), is a style of a cappella, or unaccompanied vocal music characterized by consonant four-part chords for every melody note in a predominantly homophonic texture. Each of the four parts has its own role: generally, the leadsings the melody, the tenor harmonizes above the melody, the bass sings the lowest harmonizing notes, and the baritone completes the chord, usually below the lead. The melody is not usually sung by the tenor or bass, except for an infrequent note or two to avoid awkward voice leading, in tags or codas, or when some appropriate embellishment can be created. Occasional brief passages may be sung by fewer than four voice parts.

so what is X and Y?