57 A robot hand for stacking boxes V, Z comprises pivoted arms F 1 , F 2 having hook portions k 1 , k 2 which engage flanges i on a box to lift the box. The arms F 1 , F 2 are rotated apart to drop the box V in position on a lower box Z. By positioning the hook portion k,, k 2 at different heights the box V is held at a slant so that a first lower corner of a projection 14 of the box V can be engaged with the underneath box Z, internally of the flange t,, before releasing the suspended box V by rotating the hook F 2 . Hook F 1 serves to limit sideways movement of the box as hook F 2 is released. The invention provides for more positive location of boxes during stacking without the vibration and shock experienced in the prior art method where both hooks are released at the same time to drop the box, the hook portions being at the same height.