The thumb is key to being agile on the fretboard. The thumb should be square on the back of the neck. It should not wrap round even a little. Someone sitting directly in front of you should not be able to see any of your thumb. The reason for this is that your thumb provides a stable and solid base for the fingers to fret against. The thumb should be pushing in the exact opposite direction your fingers are pushing on the fretboard. If it is pointing at an angle away, the neck is less stable and more likely to slip. Also the fingers can accidentally stretch the strings and change the note you are playing. This effect can be desirable in many situations but it should always be under control.
The active fingers are the fingers which are actually pressing down on strings at any one time. The position of these is very important. The final joint should be as close to perpendicular to the fretboard as possible and as close to the fret wire as you can manage without actually touching it.
I find that a little nail is useful on my left hand since it provides support for the rest of the finger but you should always aim to be making the contact with the fleshy tip of your finger. This gives a much smoother tone and lessens the hammer effect of banging your fingers so hard onto the fretboard that they create a note without any right hand action. Often the hammer and snap are desirable but, as with everything else to do with the left hand, control is so so important. You only want to be hammering notes when the music tells you to.
The inactive fingers are the ones that aren’t fretting strings at any given time. They should be hovering as close to the fretboard as possible without touching the strings. It is unlikely that any finger is going to be inactive for very long so it should always be in a position where it can fret a string quickly. It also looks a lot nicer for someone watching the artist. The hardest finger for me to control in this way was my little or fourth finger. It is so important to be able to control this finger and it took a lot of practice and concentration to reign it in but it is totally worth it. A fourth finger which is under control is one of the greatest assets a classical guitarist can have as it makes longer stretches and intricate holds possible and, with practice, easy to do quickly.
As with the active fingers, the final joint of the inactive fingers should be as close to perpendicular to the fretboard as possible. The actual fretting should come from the first finger joint and the angle between the second and final joint should hardly change through fretting and releasing the strings.
Each finger should be able to move independently of the others on your hand. This is especially hard considering your fingers often share tendons but with a lot of practice it is possible to get close to total individualization. It is something that every guitarist should work on every time they practice. Here is an example of an exercise:
(6i) 4 2 3 1 (5i) 4 2 3 1 (4i) 4 2 3 1 (3i) 4 2 3 1 (2i) 4 2 3 1 (1i) 4 2 3 1
Where the bracketed number and letter represent the string and the position in Roman numerals. (6i) represents first position on the sixth string. This pattern should be repeated up and down the strings. Take time with every note you fret and make sure that it is as close to perfect as possible, each and every time. I mentioned before that being in a zen type meditative frame of mind when doing these exercises is the best way and this is no exception. It is harder than the last one for me and requires a great deal of concentration to make sure everything is as perfect as it can be.
I can guarantee your hands won’t thank me for this exercise at first but persevere. At least 10 minutes every practice session along with the other exercises presented in the previous article and you will be well on your way to having a strong and dexterous left hand to match your finely tuned right hand.