C-sharp major
According to Hamasaki, the melodies composed by her staff for "M" did not fit her image for the song because they were too "warm"—she had envisioned a melody with a "cold" feeling. That, according to Hamasaki, meant a melody that was "difficult to grasp" and started in a lower key before progressing to a higher key. The song is written in common time and begins in the key of C major before progressing to the key of C-sharp major. The song uses piano, electric guitar, triangle, and various stringed instruments.