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High-resolution transmission electron microscopy (HREM) of { 111 } precipitates in an A1-Cu-Mg-Ag alloy has been used to confirm by direct observation down (110) and (211) A1 matrix zone axes that the structure of these precipitates in peak-and over-aged material is consistent with the monoclinic structure proposed by Auld [Acta Cryst. (1972), A28, $98] of a =b= 4.96, c= 8.48 A, y= 120 ° , rather than the hexagonal structure with a=4.96, c= 7.01 A proposed by Kerry & Scott [Met. Sci. (1984), 18, 289-294]. Reexamination of the monoclinic structure suggested by Auld shows that the structure he proposes is in fact orthorhombic (a=4.96, b=8.59, c= 8.48 A), and is best regarded as a distortion of the structure of tetragonal 0-AI2Cu precipitates found in over-aged A1-Cu alloys. A detailed reanalysis of electron diffraction patterns from this alloy in the light of HREM observations confirms that this structure and the relative thinness of these preciptates perpendicular to the {111} planes can indeed together satisfactorily account for the extra spots and streaks in the patterns.