The body of the guitar is the doctrines because it is the base, and without the base, there would be no support for the rest of the guitar to exist.

The neck/ headstock of the guitar represents the principles. The neck and headstock are supported by the body (doctrines) and provide support to the strings (tools).

The strings are the tools. The strings are the tools because we are acting by strumming them and we use them to share/voice the principles and doctrines with those around us.