Several songs feature extended instrumental improvisation. Bo Diddley's "Here 'Tis" and the Isley Brothers' "Respectable" are high-energy tunes, which represent the use of double-time feature of the rave up for the entire songs. AllMusic critic Matthew Greenwald describes "Here 'Tis" as "driven by a furious "Bo Diddley" beat and rhythm ... Clapton's interplay with bassist Paul Samwell-Smith is one of the great moments in the band's recorded history". The instrumental spotlight was also shared with singer and blues harmonica player Keith Relf. Clapton and Relf trading riffs is one of the highlights of "Smokestack Lightning". The Howlin' Wolf song was the Yardbirds' most popular live number and a regular in their sets. Performances of the song could last up to 30 minutes. Howlin' Wolf reportedly referred to the group's 5:35 album version as "the definitive version of his song".