AbstractAnalysis of the crystallography of the parallelogram-shaped loops observed by electron microscopy in quenched aluminium 1% magnesium reveals that these loops lie on {012} planes, and have sides along ⟨312⟩ directions. The Burgers vector of the loop dislocation is (a/2)⟨110⟩ and lies 18° from the pole of the loop. The point defects forming the loop lie on three adjacent {012} planes and the number per unit area is greater than on a {111} plane (2·69/a ², cf. 2·31/a ², where a is the lattice constant). This type of loop is not confined to quenched aluminium, having also been observed in platinum.The loops can form from hexagonal {111} loops by the elimination of the pure edge sides, followed by rotation of the remaining sides on two {111} slip planes to the {012}⟨312⟩ configuration. Presumably dissociation into partial dislocations confines movement to these planes. The energy of the loops is discussed.