Question: How can I transpose a hymn in any key into the key of C? My friend plays the organ by ear and can only play in the key of C, so I need to know how to transpose to the key of C... thank you!
Albert's reply: It's essential to be able to play in all keys! It's actually not that difficult if you learn the fundamentals. Let me give you a formula that will help both you and your friend.
As far as transposing is concerned, the reason it's so important to know all your scales is so you can determine the scale degree of each note in your music. A note's scale degree is nothing more than its position within the scale. In C major, for example, C is scale degree 1, D is scale degree 2, E is scale degree 3, etc. Once you know all your scales cold, transposing won't pose nearly as much of a problem for you.
In addition, you should learn the most common chord progressions in all keys. This will make transposing far easier, since you will encounter these chords and chord combinations over and over in most of the pieces you play.
The key to transposing is not to think in terms of the interval from the original to the new key. (For example, if you're transposing from G major to C major, don't think down a perfect fifth.)
Instead, relate every note of the piece to its tonic, the first scale degree. Using G major as our example, G is the tonic. The second note, A, is scale degree 2; B is scale degree 3; and scale degrees 4 through 7 are C, D, E and F-sharp.
Then simply play the scale degrees in the new key. For example, if you see B in the original key of G major, you should recognize that it's scale degree 3. If the new key is C major, scale degree 3 is E.
This may sound overly complicated, but the truth is that once you're comfortable with scales and common chords in all keys, and once you start transposing a few things starting with simple melodies, you'll quickly get a feel for transposing. Practice transposing simple pieces and it will eventually become quite intuitive. Transposing is one of my favorite ear training exercises, and it's one of the best ways I know to check whether you really know a piece from the inside out or whether you're merely relying on muscle memory.