Seventy-five years ago, a convincing demonstration that the genes were physically aligned along the chromosome was lacking. Harriet Creighton (1909-2004) and Barbara McClintock (1902-1992) [Creighton, H. B. & McClintock, B. (1931) Proc. Natl. Acad. Sci. USA 17, 492-497] showed by an elegantly simple experiment in 1931 that exchange between genes was accompanied by exchange of cytological, i.e., physical, parts of chromosomes. The work has been acclaimed as one of the great experiments in biology. Creighton's doctoral dissertation under McClintock's mentorship provided the basis for the landmark paper, which was unique in merging cytological with genetic data. A companion paper by McClintock, printed and bound back-to-back with the joint paper, set the essential stage with data on the cytological and genetic features that Creighton applied. Following directly from this work, and leading to today's recognition that the genome is a graspable entity, was the knowledge that the genes could be studied as components of a linear structure, the chromosome. Here, we review the data surrounding the Creighton and McClintock paper and provide a perspective on the significance of their findings.