Blue britishshorthairs are sometimes incorrectly referred to as 'Carthusians'. However, the Carthusian or Chartreux is not a British cat, but a recognized French cat breed. The cats have similarities, but the Chartreux has its own breed standard that differs from that of the British in several ways. For example, the ears are larger, they are straighter on the head and taper to a point. In addition, the head shape is not completely rounded, but more angular, with the shape of an inverted trapezoid.
The chartreux is therefore not a blue British shorthair but a French cat breed.