A violinist's left hand needs to be shaped from the beginning. If you work with very young children, as I do, ages 4 and up, then the hands are different than, a teenager or an adult.
The tiny child hand is usually chubby and does not have the knuckle action under complete control yet.
Setting the hand on the violin in playing position and learning how to place the fingers on the strings can be a daunting task for a 5 year old...or maybe even a 25 year old.
The first knuckle creates almost a square, the second finger reaches out slightly, the third finger will not square at the knuckle near the fingernail and the 4th finger will vary as the student learns to reach for 4th either by extending the finger and/or moving the arm under the violin.
The left arm plays an important role in how the fingers contact the strings. As the hand moves to the G string the arm will move under the violin.
Adult arms vary in length and so do fingers. There is no only one way to place the fingers but there is a "best way". Long fingers will place a bit differently and I am certain we have all seen great performers who place their fingers a bit differently on the strings, at least it appears that way...but there is one
common factor.....they have the "pad" of the finger on the string. For different colors and tones a bit more or less of the pad of the finger.
A lot of emphasis is placed on putting the fingers onto the fingerboard. I would like for you to also think about how you take them off.
If you have ever used an old typewriter, the action of the key as it strikes the paper is a good example.
The wrist/arm section of the left hand does not touch the violin. The wrist does not protrude out
but if observed from the side a straight line can be visualized. As the fingers move from one string to the next, the arm will move back and forth under the violin.
Poor left hand position will effect tone, intonation, and vibrato. Faulty placement of fingerpads will not allow for accuate intonation. Tone colors and tone can be effected with the fingerpads. Locked knuckles do not allow for a free vibrato.
In the table below, I have added some pictures of violinist's left hands. Some are my student's, some are professional's and some are teacher's hands. I have added the ages for reference.