Medieval Spanish had a complex system of strident obstruents. Slightly divergent paths of evolution of these segments have resulted in the major phonological differences between modern Castilian and Latin American dialects. These widely studied sound changes are here re-interpreted as changes in the sets of phonological rules of Castilian and American dialects. The role of the theories of markedness and linking, especially with respect to the features [distributed] and [strident], is discussed in some detail, and seems to provide insight into puzzling problems. In passing, it is seen that, contrary to popular belief, there has been no merger of [θ] and [s] in Latin American dialects as [s]; rather, Castilian diverged from other dialects in replacing [s] with [θ].