Tuning Down

This is a 1.37 minute video response from David Wallimann on Jun 8th, 2010

Question:

"Perhaps one day Ill realize this question is dumb and feel like an idiot for asking but for now, as a new player, I dont really understand the purpose of tuning down. Lets be sure I have the concept correct to start off with. Tuning down one step makes an E string into a D, A becomes a G ... I presume this subsequently causes chords to tune down as well. What exactly is the advantage of this? Why not just play the chords/notes as they are ( an E when you want an E...) ? Confused but Teachable"