In Classical Guitar Sitting positions I discussed how guitar supports can help ease discomfort while playing guitar and in Classical Guitar Supports I discussed the three different types of classical guitar supports available in the market and the pros and cons of each of the three designs.
In this article, I review one of the supports that is widely known and widely used, the Efel.
You'll need to go a little deeper into music theory as you proceed. The best online resource for this is Guitar Theory Revolution. It recognises that music theory is very hard for guitarists because music theory has till now been located in the piano paradigm. The Guitar Theory Revolution overthrows the piano paradigm that is holding you back and embraces the attributes of the guitar to unpack music theory. In fact, it goes further and allows you to see that the guitar is one of the best instruments for learning theory.