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short communications
In the paper by Suh, Suh, Ko, Aoki & Yamazaki [J. Appl. Cryst. (1988). 21, 521–523], the penultimate sentence of the Concluding remarks section is in error. The correct sentence is Subtract the deviation in the bottom half from the deviation in the top half to give the correction for the vertical arc: if the result is + Δν mm, rotate the vertical arc by Δv° clockwise, and by Δν° anticlockwise in the case of −Δν mm.