Beethoven developed the string quartet to the point where many consider it reached its peak of perfection. His mastery of the form is exemplified in the Opus 132 quartet in A minor, a late quartet whose slow movement is a song of thanksgiving by the composer for recovery from a period of illness. The intensity of the quartet creates a new sound world which was to influence many later composers. To his cycle of quartets, Beethoven added a transcription of one of his best-known piano sonatas, No. 9 in E major, Opus 14.