AN acute angle appears to be less acute than it really is1,2. The effect has an obvious similarity to the tilt after-effect, the only difference in procedure being that the lines forming the acute angle are presented simultaneously in one case and successively in the other. Nevertheless, Blakemore et al.2 consider that the effect they studied is not the tilt after-effect, on the grounds of different temporal properties: their effect builds up and dissipates very rapidly, which, they argue, is inconsistent with known adaptation phenomena, such as Gibson after-effects3,4, which have long time constants.