Complete independence of the left hand fingers is a laudable goal for every serious bassist, guitarist and string player. Below is a short beginning exercise designed to prepare us for playing double trills, (trills on two notes simultaneously).
Strictly speaking, there are no double trills in the exercise below. However, it is a necessary step we must take if we wish to deftly and effortlessly execute double trills. Even if we never play double trills, mastering this exercise will improve the coordination of our left hand fingers substantially, which is always a good thing!
Remember never to use excess muscular force. Stay supple and light.
* To be played on the G and D strings. The “cross” symbol indications use of the thumb.
Of course you can, and should, adapt this exercise to be played with all the finger combinations you can discover, on all sets of two adjacent strings, and in both the lower and higher positions.
Practice, repeat, and enjoy!