Learn how to use open strings in melodies, chords, and arpeggios when playing in a variety of flat keys. When guitarists talk about guitar-friendly keys, it's almost always the sharp keys C, G, D, A, E all guitar friendly and all with sharps in their signatures, not flats. Some flat keys are called horn keys and are no fun for guitarists. Or are they? Because one of the things that makes something easy on the guitar is the presence of open strings, let's take a closer look at a few flat keys and see how many standard-tuning open-string notes can be found in them.