Throughout the first movement the harpsichord part also has several episodes with "perfidia"—the same half bar semiquaver patterns repeated over a prolonged period. Both outer movements are in an "A"–"B"–"A′" form: the "A" section of the first movement is in bars 1–62, the "B" section starts with the bariolage passage and lasts from bar 62 to bar 171, the "A′" section lasts from bar 172 until the end; the "A" section of the final movement is in bars 1–84, the "B" section in bars 84–224, and the "A′" section from bar 224 until the end. In the first movement the central section is in the keys of D minor and E minor; in the last movement the keys are D minor and A minor. As in the opening sections, the shifts between the two minor tonalities are sudden and pronounced. In the first movement Bach creates another equally dramatic effect by interrupting the relentless minor-key passages with statements of the ritornello theme in major keys. Jones describes these moments of relief as providing "a sudden, unexpected shaft of light."