The tritone plays a key role in the Symphony in A minor. Since the medieval theory of the octoechos, the tritone occupied a specific position among the other intervals. Within the tonal system, it remains the main dissonant interval, both "destructive" of tonality as carrier of instability and tension, and carrier of the tonal affirmation in the dominant chord. Sibelius, however, utilizes it in a mobile way that blurs tonality. When the tritone is built upon the tonic, only the ascending leading tone finds resolution on the fifth, while the tonic itself resists the descending attraction. The first theme of the Symphony op. 63, presenting from the outset the interval of the augmented fourth as the key interval, imparts the aesthetic power of the whole work. One finds it therefore as characteristic element of the initial theme of both the second and the last movements. The Fourth symphony so gains a unique position in the production of the composer: Sibelius, with this singular work, goes largely beyond the mere role of Finnish composer among the European composers of the first half of the 20th century.