AbstractThe classical phenomenon of crossover interference is a one-dimensional spatial patterning process that produces evenly spaced crossovers during meiosis. Quantitative analysis of diagnostic molecules along budding yeast chromosomes reveals that this process also sets up a second, interdigitated pattern of related but longer periodicity, in a “two-tiered” patterning process. The second tier corresponds to a previously mysterious minority set of crossovers. Thus,in toto, the two tiers account for all detected crossover events. Both tiers of patterning set up spatially clustered assemblies of three types of molecules (“triads”) representing the three major components of meiotic chromosomes (crossover recombination complexes and chromosome axis and synaptonemal complex components), and give focal and domainal signals, respectively. Roles are suggested. All observed effects are economically and synthetically explained if crossover patterning is mediated by mechanical forces along prophase chromosomes. Intensity levels of domainal triad components are further modulated, dynamically, by the conserved protein remodeler Pch2/TRIP13.