Ex. 1 is one of the most recognizable double-stop sounds—ain’t it funky! Play the root of the starting chord, E7. Then play the G# on the G string with your 2nd finger while grabbing the D with your 1st finger on the D string. You are playing the 3rd (G#) and the b7th (D) of the E7 chord. Change to the A7 chord on beat three, and slide your two fingers down one half-step to the b7th (G) and the 3rd (C#) of the A7 chord. Proceed through the cycle in the next bar, moving the roots up in 4th intervals and playing the 3rds and b7ths or b7ths and 3rds in the double-stop on top. You can precisely describe the quality of a chord by outlining the root, 3rd, and b7th.
Jaco Pastorius was a master of outlining complicated harmonies by using double-, triple-, and even quadruple-stops. His version of the Beatles classic “Blackbird,” off Word of Mouth [Warner Bros.], serves as a perfect example of double- and triple-stops harmonizing a melody and outlining chords. Ex. 2 is similar to the open-string A pedal he plays while changing from the Amaj7 to the triple-stop chord on the Dm7/A. Since we’re using three notes, we can officially call this a chord.
Even in the early days of bebop, Oscar Pettiford was no stranger to using chords on the bass. Example 3 is similar to the major-7 chords Oscar strums in his cadenza to his 1955 recording of “Stardust” [Another One, Rhino], his favorite ballad feature number.
When played live by one bass player, Ex. 4 falls into the party-trick category. The line sounds like the two harmonized bass lines originally multi-tracked in 1972 by acoustic/electric studio-ace Herbie Flowers on Lou Reed’s “Walk on the Wild Side” off Transformer [RCA]. The acoustic bass plays the bottom note, sliding down from the root C to the root F and back up again. The bass guitar plays the upper voice, playing the 3rd of the C chord (E) and sliding up to the 3rd of the F chord (A). While technically not a double-stop (because it was overdubbed on two separate tracks), this bass line could be played by a bass guitarist who can tap with both hands.
Go for it—double your fun, triple your pleasure by using double-stops and triple-stops, but be tasteful.