The thumb should be touching the neck roughly on the pad, just below the nail. Without Squeezing the violin should just rest between the wedge made by your thumb and side of your index finger. There should be a space big enough for a finger between the webbing of your thumb and the neck. The wrist should be straight.
How should your thumb be when holding a bow?
Keep your thumb bent down to the hair or the bow might roll away from you. Without upsetting the balance, lightly lower your fingers to the stick as the fingers fall naturally—the first three over, the pinkie tip on top. Your forearm will likely rotate inward. That should be a light, relaxed bow hold.
How can I make my bow hold better?
Long bows in the air Holding the bow in playing position above the string, but as close to the string as possible, do whole bows down and up, as slowly as you can. Practiced for a few minutes daily, over time you will notice greater strength and control in your bow hold.