In an analysis- by-synthesis investigation of music performance, rules have been developed that describe when and how expressive deviations are made from the nominal music notation in the score. Two experiments that consider the magnitudes of such deviations are described. In Experiment 1, the musicians' and nonmusicians' sensitivities to expressive deviations generated by seven performance rules are compared. The musicians showed a clearly greater sensitivity. In Experiment 2, professional musicians adjusted to their satisfaction the quantity by which six rules affected the performance. For most rules, there was a reasonable agreement between the musicians regarding preference. The preferred quantities seemed close to the threshold of perceptibility.