ii - IV - I. B flat diminished, flat 5. How many sharps in the key of F#? (trick question, that...) So, is it important that you know music theory? To answer the question, I think you need to ask where you are trying to get to with your musical career? Dubbing around with friends at a party? Joining a band and play local bars and pig roasts? Or are you looking to make real money and get real respect as a musician? If you answered yes to the last one, then you better be able to transpose down a minor third at the drop of a hat when the singer in the session cant reach the high notes. And the faster you can make that shift, the faster you will be hired back for the next session. The people who hired you will not be generous with the time and money it takes for you to figure it out. And if that's not reason enough, then consider this... When you are playing in the key of D, and you tell a fellow musician to play a G flat (instead of an F#), you just lost a lot of respect. As my friend Anthony said to me: "It's like saying 'I is a bass player'. People know what you mean, but they are not going to think highly of your communication skills"