What instrumental key is the guitar in?

There is no single instrumental key that a guitar is in, as guitars can be tuned to a variety of different keys and scales, depending on the preferences of the musician or the requirements of the piece of music being played. Guitars are most commonly tuned to the standard E tuning, which is E, A, D, G, B, E, from the lowest string to the highest string, but alternate tunings are also possible.