As part of a collaborative project between the Staatliches Institut für Musikforschung and the Audiocommunication Group of the Technical University Berlin, we measured bar by bar tempi in the first movements of three of Ludwig van Beethoven’s piano sonatas (Appassionata Sonata, Sonata op. 2 No. 3, Hammerklavier Sonata) in interpretations from the 1920s through the 2000s. Beyond the questions whether tempo and tempo design changed over time, whether there are national or culture-specific traditions or whether inter-subjective tempo decisions can be identified, we specifically compared the tempo indications in Artur Schnabel’s edition of the Beethoven sonatas (1924-27) with the tempo design in his recordings (1932-35). While it has to be admitted (against repeated claims) that Schnabel’s differentiated tempo designs very often correspond to the sophisticated tempo indications in his edition, there are nevertheless some notable discrepancies. These discrepancies not only reflect the difference between a more distant, coolish calculation of tempo in Schnabel’s theory of performance and the more suspense-packed, intense atmosphere in the concert hall or the recording studio, but they also represent two competing concepts of musical form: an ‘architectural’ ideal of form and an idea of form as process. In his performances Schnabel realized an evolutional concept of form to an extent he didn’t advance to in his performance theory.