A detailed definition of feather-fractures is given. Studies of joint planes in rocks of various lithologies and ages in the coal fields of south Wales illustrate the fact that feather-fractures occur most frequently on shear joint surfaces in fairly fine-grained rocks. The mode of origin is discussed and comparison is made with the "cleavage fracture" of mild steel which is caused by brittle fracture. From the detailed study, it is possible to attempt a reconstruction of the actual centers of initial fracture.