Mean period of the RRLs pulsating in the fundamental mode as a function of the iron abundance of the cluster. The dotted lines indicate the region known as the Oosterhoff gap, we can see that multiple clusters are located in this region. The error bars represent the 16th and 84th percentiles of the distribution of periods. The blue point represents NGC 6388, and the yellow one corresponds to NGC 6441. Both clusters have been classified as Oosterhoff type III in the literature (Pritzl et al. 2003; Bhardwaj 2022).