Melodies (the tune of a song) are made from scales, and sometimes purely from scales.
In “The First Noel”, for example, the melody is made entirely from the diatonic scale. I demonstrate it in the key of C, but the same would be true in any other major key. Watch this short video:

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Music is simpler than most people make it out to be. There are only scales, scale fragments, intervals, chords, and rhythm. Look for the lowest possible denominator whenever you can. Ask yourself “Is this song made out of a scale? Out of a chord?” In this old Carol, you can see that the melody is nothing but the scale for that key — and only one octave at that.

The First Noel The Angels Did Say