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Questions we've been asked

If you transpose a piece a major 2nd up or down, does it mean that the original piece (which needs transposing) is in a major key .... and should be transposed into a major key, or could it initially be a minor key?  Confused. (10/10/08)

Mymusictheory says: If your piece is in a major key, then it will stay in a major key when transposed- it doesn't matter what interval you transpose it by. So, for example, a piece in D major (with 2 sharps) which is transposed up a major second, it will then be in E major (4 sharps). If you transpose it down a major second, it will be in C major (no flats or sharps of course!). If you started in D minor (1 flat), going up a major second would give you E minor (1 sharp), and going down a major second would give you C minor (3 flats). An interval of a major second is equal to 2 semitones - it doesn't matter whether the piece is in a major or minor key. A piece in any key can be transposed by any interval.

For the tranposition questions, you don't actually need to work out whether the key is major or minor. Just look at the key signature, and assume it's a major one, and work from there. For example, let's imagine the piece has 2 sharps, but we're not sure if it's D major or B minor. Let's assume it's D major. If you transpose up a major second, you need to put it into E major (4 sharps). Then transpose each individual note in the piece up a major second. If, in fact, the piece was really in B minor, then it should now be in C sharp minor - which also has 4 sharps! So, you can see it actually makes no difference. It's easier to assume it's a major key, as then you don't need to do much extra thinking!