It has been described as "completely original in form, subtle in its handling of tempi, individual in its treatment of key and wholly organic in growth" [3] and "Sibelius's most remarkable compositional achievement".

The overall key seems to have been G minor, while the second movement, an Adagio in C major, provided much of the material for the themes that eventually made up the Symphony.

[6] The first surviving draft of a single-movement symphony dates from 1923, suggesting that Sibelius may have made the decision to dispense with a multi-movement work at this time.

[7] When he returned to the symphony, the composer drank copious amounts of whisky in order, he claimed, to steady his hand as he wrote on the manuscript paper.

7 was Sibelius's final home for material from Kuutar, a never-completed symphonic poem whose title roughly means "Moon Spiritess".

[citation needed] But in response to this symphony, the British composer Ralph Vaughan Williams said that only Sibelius could make C major sound completely fresh.

Peter Franklin, writing of the Seventh in the Segerstam–Chandos cycle of Sibelius symphonies, calls the dramatic conclusion "the grandest celebration of C major there ever was."

Since the time of Joseph Haydn, a movement in a symphony would typically be unified by an approximately constant tempo[citation needed] and would attain variety by use of contrasting themes in different keys.

The symphony is unified by the key of C (every significant passage in the work is in C major or C minor), and variety is achieved by an almost constantly changing tempo,[11] as well as by contrasts of mode, articulation and texture.

At the climax, the first trombone announces the main tune of the symphony (bars 60–64), labelled "Aino" in sketches, after the composer's wife.

The key signature switches to C minor: Soon the tempo is ratcheted up to Vivacissimo (very lively), with fast staccato chords traded between the strings and woodwind.

The music turns stormy in mood with ominous ascending and descending scales on the strings, while the "Aino" theme is heard again in the brass: The symphony contains the following tempo markings at these points in the score: This section ends with a chord progression from A♭ back to the symphony's main key of C major taken directly from Sibelius's earlier work Valse triste from Kuolema.

The strings play a version of the theme from bars 11–12 against a grand C major chord held by the brass and woodwinds.