Recent studies have shown brain differences between professional musicians and non-musicians with respect to size, asymmetry or gray matter density of specific cerebral regions. Here we demonstrate: (1) that anatomical differences in the motor cortex can already be detected by coarse visual inspection; and (2) that within musicians, even a discrimination of instruments with different manual dominance is possible on a gross anatomical scale. Multiple raters, blinded for subject identity and hemisphere, investigated within-musician differences in the Omega Sign (OS), an anatomical landmark of the precentral gyrus associated with hand movement representation. The sample of 64 brains comprised matched groups of 16 expert string-players, 16 expert pianists and 32 non-musicians. Ratings were analysed by means of kappa statistics. Intra- and interobserver reliabilities were high. Musicians had a more pronounced OS expression than non-musicians, with keyboard-players showing a left and string-players a right hemisphere advantage. This suggests a differential brain adaptation depending on instrument played.