The concept of a continuous, single-movement symphony was one Sibelius only reached after a long process of experimentation. His Symphony No. 3, dating from 1907, contained three movements, an earlier fourth movement having been fused into the third. The final result was successful enough for Sibelius to use the same idea in his Symphony No. 5, completed in 1915. Although his first mention of No. 7 occurred in December 1918, the source for its material has been traced back to around 1914, the time when he was working on the Fifth.