How do tetrachords combine to form a major scale?

In Western music, a major scale is constructed by combining two disjunct tetrachords—four-note groups that do not share a common tone.1

Each of these tetrachords follows the same internal interval pattern: a whole tone, a whole tone, and a semitone. When these two groups are joined, they form the seven-note diatonic structure of the major scale. For example, in the scale of C major, the first tetrachord (C–D–E–F) and the second tetrachord (G–A–B–C) are separated by a whole tone, resulting in the complete octave sequence. This arrangement ensures that the half steps in the scale occur specifically between the third and fourth degrees and the seventh and eighth degrees.12