Characterization of precipitates in Al-Zn-Mg alloys, using a combination of electron diffraction, bright field transmission electron microscopy and atomic scale scanning transmission electron microscopy imaging revealed the presence of an unreported η orientation relationship between the η-MgZn phase and the Al lattice with the following orientation relationship (0001) ∥ (120) and (21¯1¯0) ∥ (001) , plate on (120) . The precipitate interfaces were observed and analyzed along two projections 90° to one-another. The precipitate coarsening was through the common thickening ledge mechanism. The ledges were significantly stepped along one lateral direction. An interface relaxation model using density functional theory was carried out to explain the precipitate behavior. 13 2 η Al η Al Al