Any piece of music may be composed in one key (scale) and played in another. For example, a song written in the key of C major, may be played in A major, F major, D major, etc., as one chooses. It may be done for the sake of ease of play in a particular key. A song is easier to play in using the notes and chords of C major scale than in C# major. Transposition may also be done to suit a song to the vocal range of the singer.
Moreover, if you are a new played and find a song in a difficult scale (key), you can always transpose the song up or down a few semitones to the scale you are comfortable with. You can later come back to the real key and learn it when you are skilled enough.
Song in C major scale:
C F C We shall over come....
Song transposed to Eb major scale (transposed up with respect to C major):
Eb G# Eb We shall over come....
Song transposed to A major scale (transposed down with respect to C major):
A D A We shall over come....
The property of music is that it allows it to be transposed from one key to another without altering the song. The song sounds almost the same, just more high pitched. Some songs even incorporate parts where the key is shifter up (or down) by a certain number of notes. Thus the song itself incorporates a piece of transposed music.
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