The invention relates to an open-ended mould for casting metal strands having a thin slab cross-section, in which the wide lateral walls (1, 2) form a central enlarging region (11) extending from the mould inlet edge (5) to the mould outlet edge (6), and opening up in a rectangular outlet cross-section (22). The aim of the invention is to prevent, to a large extent, strands from detaching themselves from the wide lateral walls. To this end, the contour of the mould cavity (7) in the enlarging area (11) is formed, in cutting planes which are perpendicular to a longitudinal axis (23) of the mould, by curves (15) which form, at least in partial regions, clothoid curves (18, 19, 20, 21) or clothoid-type curves, in which the curvature variations (Δk) are minimised or reduced to zero.