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An extended chord can be formed by sharpening the fifth of a dominant or major seventh chord – C7: C E G Bb
becomes C E G# Bb; Cmaj7: C E G B becomes C E G# B. This is like superimposing an augmented triad on a major
triad two tones (full steps) above. It has a mildly dissonant sound. There's a 7#5 chord at the end of each four-bar
phrase in John Lee Hooker's 'The Healer'.
This chord imparts a mild blues/jazz feel and is used as a passing chord or as a means of changing keys. The raised
fifth may lead to either the fifth or the sixth in a different chord. C7#5 might go to F or Am.