Direct structural images of a twin boundary in a superconducting Y–Ba–Cu–O ceramic have been obtained at 0·19 nm resolution by high-resolution transmission electron microscopy and an image processing technique. It was found that theaandbplanes of a twin domain gradually change to thebandaplanes of a neighbouring twin domain across a twin boundary. This involves a shifting of the respective planes by a distance (⅓∼½)d(110)along the boundary as well as the rotation of the planes by ø ≃ 2(b−a)/(a + b) across the boundary. Also, this transition takes place within approximately four times the (110) interplanar spacingd(100), which is considered as the twin boundary layer thickness.