A Quick One is the second studio album by the English rock band The Who, released on 9 December 1966. The album was also released under the title Happy Jack on Decca Records in April 1967 in the United States, where the song "Happy Jack" was a top 40 hit.

The Who's second studio album departs from the R&B emphasis of the first. Part of the marketing push for the album was a requirement that each band member should write at least two of the songs on it, though Roger Daltrey only wrote one ("See My Way"), so this is The Who album least dominated by Pete Townshend's songwriting. It was recorded at IBC Studios, Pye Studios, and Regent Sound in London England in 1966 by record producer Kit Lambert. Townshend said that this push for equal contribution led to the exclusion of the band's singles that he had written.

"Boris the Spider" was written after John Entwistle had been out drinking with The Rolling Stones' bass guitarist Bill Wyman. They were making up funny names for animals when Entwistle came up with the song. "Boris the Spider" quickly became Entwistle's most popular song, still performed decades later: in later years he often wore a spider necklace.

"Heat Wave", the only cover-version and the only reversion to the group's soul influences, a song by Tamla's Holland–Dozier–Holland team, was replaced by "Happy Jack" on the original US release but included on the 1974 double album repackaging of A Quick One and The Who Sell Out.

"A Quick One, While He's Away", the title track of the album, is a nine-minute suite of song snippets telling a story of infidelity and reconciliation, a first foray into extended form that led to the so-called "rock operas" Tommy and Quadrophenia.

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