Probability density functions based on kernel density estimations with the Gaussian kernel and adopting the critical bandwidth of Silverman (1981) of the period distribution of different sub-samples of cataclysmic variables. From the top to bottom panels, we excluded the polars, the polars plus the nova likes, and finally the non-polars. The vertical dashed lines indicate the location of the modes assuming bimodal distributions (top and middle panels) and a unimodal distribution (bottom panels).