Never squeeze the instrument’s neck with your thumb!
Remember, the instrument’s neck is a neck, not a handle! Depending on the length and shape of your thumb and the thickness of the instrument’s neck, the pin-point location of the thumb varies from person to person. Here is some handy advice:
The thumb serves to support the state of motion created by the actions your fingers perform. Creating different pitches at all regions on the strings requires thumb flexibility and mobility.
Most string players are successful when playing with a loose straight thumb, where the pad of the thumb’s tip segment makes gentle contact with the instrument’s neck.
Focus on finding the thumb placement that works well for you!
Violin and Viola: In 1st position, your left thumb is placed on the left side of the instrument’s neck and slightly under the neck. The left thumb is placed opposite your index-finger or slightly more towards the center of your hand.
Cello andBass: The left thumb is placed under the instrument’s neck, opposite your center-finger and varies location in order to increase finger mechanic leverage.
Always practice with the goal of securing a flexible, tactile sensation to your thumb mechanics.
In playing position, the tip of the thumb does not wrap over the fingerboard.
Keeping your thumb muscles relaxed, makes it possible to have increased control of your finger mechanics.