Watch the Using the Thumb online guitar lesson by David Grissom from Open Road Guitar
One of the essential techniques that is non-negotiable in my style, or if you want to become a great rock, blues and country player is to use your thumb to fret notes over the top of the neck. If you don't it's like leaving money on the table! As I discussed in earlier sections I use my thumb over the top of the neck to get strength in my bends, vibrato, and control over the guitar. Beyond that it serves many other purposes like playing the lower note in a chord as in the minor seven chord in example one. I can also use that fretted note as a pivot point to play notes above and get a really big sound that I love. It is important when playing that low note to let the thumb hang over and mute the 5th string so it doesn't sound. I will also play the the low note and the octave above if I want a really thick fat sound. There are two ways I like to use my thumb over the top of the neck quite a bit. The first one is sort of a hillbilly G chord. This is where my thumb gets the low G, mutes the A string, open D and G, and I fret the D and G on the top two strings. This gives me a G5 chord, as there is no 3rd. I like this fingering as it puts me in a good position on the neck to jump off into other ideas. The second chord is an Fadd9 and I'm only sounding 4 strings. From low to high my thumb is playing the F on the 6th string, muting the 5th, the third finger plays the F on the 4th string and hangs over to mute the third string, my first finger plays the C on the 2nd string and the fourth finger plays the G on the 1st string. If you told me I couldn't use my thumb in my playing I would be lost as it serves so many purposes for me.