Period distribution of all known mass-transferring UCBs, colour-coded according to their accretion disc behaviour in the case of AMCVn-type systems (those without detected hydrogen). The He CVs (systems with detected hydrogen) and other classes are coloured separately. The accretion disc behaviour clusters by orbital period, but with some regions of overlap between the different subclasses. He CVs are typically found at periods ≳50 min. We note that most He CVs do show outbursts, while systems in the same period range without hydrogen typically do not. The orbital period distribution is strongly influenced by selection effects, most notably the fact that it is easier to find outbursting systems.