Thumb Placement for Violin and Viola
The thumb should be lengthened and straight, but not locked or stiff. The thumb should never be hyperextended. One way to achieve this position is to have students hold instruments in front of their bodies in rest position and ask them to "shake hands" with the neck of the instrument.
There are many viewpoints about the "exact" placement of the thumb in relation to the neck and fingerboard, but since thumbs come in varying sizes it is difficult to give an exact placement. If the fingers are naturally curved, over the fingerboard, and the fleshy point of the fingertip of all four fingers can make contact with the string, the thumb is likely in a good position.
On viola, the same principles
hold true as for the violin, but some violists bring the thumb closer
to center of hand to help with the longer stretches required. If you
are interested in this topic, the great violist William Primrose discusses
the differences between violin
and viola technique in the book Playing the Viola : Conversations with
William Primrose by David Dalton.