These are heptatonic (seven note) scales that traverse the octave in the following pattern of steps with respect to the chromatic scale:
2 - 2 - 1 - 2 - 2 - 2 - 1
The above series indicates that after playing the tonic note, the first note of the scale (akin to the root note of a chord), after which the major scale will be named, we play the second note, which is 2 semitones away from it. The third note of the scale is 2 semitones away from the second note, and so on and so forth.
Let us take the example of the commonest (major) scale, C major:
The figure above shows the notes of the C major scale, the C ascending chromatic scale and the step differences between the notes (2212212) marked in red. Thus the C major scale was formed by choosing the notes starting from the tonic note (here, C) and following the map of 2212212 semitone jumps. This same principle applies for all other major scales.
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