At the dawn of investigations into RR Lyrae stars in globular clusters, observations by Solon Bailey demonstrated that RRab variables in different globular clusters occupied different loci in the period-amplitude diagram. It was much later before it was realized that the properties of cluster period-amplitude diagrams were correlated with Oosterhoff type and with globular cluster metallicity. Recent observations let us revisit the question of which parameters determine the location of the RRab period-amplitude diagram. We are closer to determining what are the rules governing the period-amplitude diagram, and what are the exceptions to those rules.