Firstly, look at the key note and decide what you will change it to. For example, if the key note is D and you want to raise it to E, you’ll need to move all of the notes in the tune up two semitones.
The type of key or mode (mixolydian/major etc) will not change. That is, when transposing a major key, it will stay major. A mixolydian key will stay mixolydian etc.
But the key signature will alter – see the table below. For example, if the key of your tune is G Dorian and you move up to A Dorian, the key signature will change from 1 flat to 1 sharp.
To illustrate – the example below is in E dorian.
Now I’ll move all the notes up four steps on the stave. The first note is now B. Another sharp note has been added to the key signature to make B dorian.
And here, I’ve changed the E dorian original down to D dorian. The notes have all moved down a step on the stave and the key signature displays no sharps or flats.
If you want to be more adventurous, you can change a tune by keeping the notes on the stave the same, but changing the key signature. This isn’t transposing – it’s actually changing the mode of the tune so it’s still familiar, but just has a different feel to it. It’s good fun to try out on a tune you know well. Compare the following examples:
I hope all this theoretical stuff has been useful. I’ve tried to keep it relevant to the folk player – the information applies to most traditional music from the British Isles and North America and large parts of Europe, too. When you move into Asian or Middle Eastern music for example, you find a whole world of different modes, but the fundamentals of transposing stay the same.