The Op. 76 quartets are among Haydn's most ambitious chamber works, deviating more than their predecessors from standard sonata form and each emphasizing their thematic continuity through the seamless and near-continual exchange of motifs between instruments. In addition to not using the expected sonata form in some of the string quartet's first movements, Haydn employs uncommon forms in other movements such as a canon, a fantasy and an alternativo. He also plays with tempo markings, key signatures and many sections emphasizing the viola and cello. Charles Burney wrote to Haydn praising these innovations: