This ActionTab is from the Theory section on Alternate Tunings.

Drop D tuning is not just for metal. It is quite common in more acoustic guitar styles too.

Here we use the thumb to play basslines in Drop D Tuning. This is with classical guitar style, but applies across other styles too. This simple little tune would be much harder to play just using fingers.

To play with your thumb involves curling your thumb over the top edge of the fretboard and onto the string to fret bass notes on the Lowest string. Meanwhile your fingers come from beneath to play notes in the usual way. This thumb technique is not for everybody - some people find it painful or too awkward. If this is the case, don't push it and move onto something else.

You can use your thumb to fret notes along the Low D string whilst your fingers hold notes along the fretboard as usual. This gives you some more musical options when constructing those lovely D-focused basslines for your music. This is just a simple example.

Drop D is a nice tuning with some useful advantages. Experiment with it in different musical styles and you'll find it surprisingly useful and fun!