A lot of Hendrix's songs are built around the 7th#9 chord (or the Hendrix chord). The reason this chord has such a distinct sound is because it has both a major and minor 3rd. Western music has chords mostly built on major and minor 3rds stacked so C major is made up of C, E, and G. E and C are 4 half steps apart which is a major 3rd and G is a 7 half steps up from C which is a 5th. minor 3rds are 3 half steps. then if you add a 7th it's a major or minor 3rd up from the 5th and a 9th a 5th up from the 5th. But a 7th#9 has a lowered 7th by one step and a raised 9th by one step giving it elements of both major and minor chords spread out just enough to not sound too dissonant but still together enough to add a distinct bluesy depth.