I've been playing electric bass guitar for 30 years, and I started playing double bass about 10 years ago. Maybe because of my experience with the bass guitar, I am in the habit of playing double bass with my left thumb located behind my left index finger, with the thumb in the middle of the back of the neck. I just took a lesson with an experienced classical bassist and double bass instructor, and she strongly suggested that I should have my left thumb behind the second finger of my left hand. I have taken lessons with several teachers over the years and have attended workshops with John Clayton and other pro players and nobody has corrected or commented on my left thumb location. After the lesson I looked in all my bass instruction books and the only clear shot of the left thumb behind the second finger is in a Ron Carter book from the 70's; all the other books that I have (Rufus Reid, Simandl, Ray Brown) show pictures of the left hand and finger placement from the FRONT view. You can infer the location of the left thumb, but I can't find any discussion or clear photos. Anyone here have suggestions or preferences about the left thumb location behind the first or second finger of the left hand?