Originally Posted by: jeff896

Every video I watched, here and YouTube, states the end of the thumb is supposed to be used as additional support on the back of the fret board. However, this is too painful for me and my chords sound aweful. The only way, so far, I can play is using the mid-section of my thumb.

In general you should use your thumb to support or anchor your hand in a way that helps your fingers fret notes. But that depends on what you are playing (which strings, which area of the fretboard, which style of music). It helps to realize that this is going to require moving your thumb & adjusting it so the general principle works but adjusted for specific applications.

JeffS65 basically covered this in his excellent reply. The 3rd lesson of this tutorial covers the idea in terms of what I call an axis of power.

https://www.guitartricks.com/tutorial.php?input=605

Hope that helps!