A close examination of two species and a survey of a further 281 species of libellulid dragonflies, representing 25% of the family and including members of all sub-families, has shown that the sub-genital plate is extended by more than a third of the length of the 9th sternite in 38% of the species examined (in a few it exceeds the length of sternite 9), by between a tenth and a third in 16%, and by less than a tenth in 45%. Extended plates are slotted in 39% of species and unslotted in 61%. During copulation the penis rides into the slot if one is present, but in species with extended unslotted plates, the penis is much lengthened to allow it to reach into the female’s genital opening. In some species with slotted plates, a lengthening of the inner branches of the hamules allows the branches to engage with specialised recessed notches situated proximally on the plates. In species with unslotted plates, such notches are more commonly placed distally, and the hamules are not lengthened. Extended plates occur in members of 6 libellulid subfamilies, and they have probably evolved several times independently. The occurrence of extended plates is most probably an adaptation for specialised modes of oviposition. The possibility that they are also significant for sperm competition or for species isolation is considered.