Three perceptual experiments using natural-sounding instrument tones arranged in concurrently sounding pairs investigate a problem of orchestration: what factors determine selection of instruments to achieve various degrees of "blend" (fusion of multiple timbres into a single timbral image). The principal finding concerns the spectral centroid of the instruments (the midpoint of the spectral energy distribution). Blend worsened as a function of the overall centroid height of the combination (the centroid of the composite spectrum of the pair) or as the amount of difference between the centroids of the two instruments increased. Slightly different results were found depending on whether the instruments were on the same pitch or separated by a minor third. For unisons, composite centroid, attack similarity, and loudness envelope correlation accounted for 51% of the variance of blend. For minor thirds, centroid difference, composite centroid, attack similarity, and synchrony of offset accounted for 63% of the variance of blend. In a third experiment, instruments were manipulated to have different centroid levels to test if centroid made an independent contribution to blend. The results show that changes in centroid affect blend even when that is the only aspect of the sound that is changing. The findings create the potential for an approach to orchestration based on abstract properties of sound as a substitute for the traditional approach of teaching entirely by example.