The thumb is an important component of left hand technique on the guitar (Classical/Electric/Folk/etc…). It is something that I work on with my own students and am constantly refining with them, and in my own playing.  I think most of the issues people have with their left hand, more specifically their thumb, comes from information they are given in their beginning lessons.

In The Beginning

Think back to the beginning of your guitar studies.  I’m sure your teacher at the time spoke about keeping the left hand thumb behind the neck of the guitar, without the tip sticking over the top of the neck.  However, other then the fact that the teacher tells the student to keep their thumb there, the student doesn’t understand why they are doing it.  The reason why every teacher tells their students to keep the thumb behind the neck of the guitar is because the left hand’s job is to fret the instrument.  With the thumb reaching over the neck, the left hand starts holding onto the neck (much like someone gripping onto a baseball bat).  If the left hand is busy holding onto the neck of the guitar, then you cannot focus completely on it fretting with the left hand.  This is why many teachers are adamant about where the left hand thumb is placed, including myself.

However, there are plenty of exceptions to the rule, so much so, that the rule itself becomes obsolete.  As I explained in the previous Using The Left Hand article, the thumb moves vertically on the neck as the left hand crosses the different strings.  In this case, the thumb staying in the middle behind the neck doesn’t work.  You have to move the thumb to keep the same angle and curvature of the fingers from string to string.  Which means at some point, the thumb may be peeking itself over the neck of the guitar.  As long as you aren’t grasping with the thumb, having the thumb peek over is ok.  A great example of this is John Williams.

How The Thumb Works

Understanding how the thumb works is essential to developing a left hand technique.  Start by opening and closing your hand.  Notice how the fingers and more importantly the thumb move toward each other:

  1. The thumb moves inward towards the center of the hand (towards fingers 2 and 3)
  2. The side of the thumb, not the pad of the thumb, touches the fingers as it comes into the hand

The left hand approaches the neck of the guitar simply by closing.  Because of this natural movement, the thumb should be placed behind the 2nd finger (depending on the persons physical make up, it may be in between 1st and 2nd or between the 2nd and 3rd).  Also, in the beginning we are taught to put our thumb on the back of the neck, but we typically aren’t taught which part.  What ends up happening is people place the pad of their thumb down and pinch the neck of the guitar.  Instead, based upon how the thumb moves, the side of the thumb should be placed on the neck.  The thumb’s primary role is that of balancing the left hand.  Placing the thumb behind the 2nd finger (rather then the 1st) and by placing it on the side rather then the pad, you avoid squeezing the neck.

In chord playing, keep the thumb in the same area as the lowest finger to allow the other fingers to move freely.  This allows you to reach for lower notes, and maintain the curl of the fingers.  This may cause the thumb to be higher then where you usually keep the thumb, however remember the thumb can move vertically (up and down) on the guitar.  This may include the thumb peeking over the neck of the guitar from time to time, and it is alright as long as the thumb isn’t wrapping itself around the neck of the guitar.

While playing barres, a big complaint is that people use too much thumb pressure when they play and start feeling pain at the base of their thumb.  I suspect 9 times out of 10 this is due to improper alignment of the thumb.  Having the thumb behind the 1st finger creates a squeezing motion between the thumb and the first finger.  Thus instead of using arm weight to help aid for a barre, it becomes more of a squeeze.  With the thumb aligned with the 2nd finger, you loose the ability to squeeze the neck of the guitar as much, which in turn can allow you to concentrate on arm weight.

There a loads of different variations to thumb placement on the guitar.  I think the important thing to remember is the general concept of the thumbs roll in technique.  Those guidelines will allow you to make the best decision as you continue to develop your technique.