Hidden within the marked idiosyncracies characterising synaesthesia is a set of cross-sensory associations common to all synaesthetes. These associations, or correspondences, bring the different sensory systems together. They were first revealed when Lawrence Marks (Yale) studied historic accounts of visual-hearing synaesthesia. He observed, for example, that higher pitch sounds, including vowel sounds, induce visual images that tend to be brighter, higher in their spatial elevation, lighter in weight, more likely to be moving, sharper, and smaller than those induced by lower pitch sounds. It would appear that high pitch sounds can, in some way, feel bright like visual stimuli, and light in weight like felt objects. Many of the same cross-sensory correspondences have now been demonstrated in the general population, including infants, confirming a fundamental, yet largely unexplored, aspect of the unity of the senses. After reviewing the essential nature and functional organisation of cross-sensory correspondences, it is illustrated how correspondences have the potential to impact on human performance (e.g., in man-machine interactions), to enhance artistic endeavours (e.g., in music), and to shape the immediate impressions we form of new products based on the sounds of their brand names.