Re-examination of the structure of Ω precipitate in Al-Cu-Mg-Ag alloys through first-principles density functional theory (DFT) calculation reveals that the widely-accepted orthorhombic structure of the ‘metastable’ Ω phase is actually tetragonal that is identical to that of the equilibrium θ phase in binary Al-Cu alloys. When the tetragonal θ (i.e. Ω) is embedded in α-Al matrix as thin plates, the elastic distortion imposed by the surrounding α-Al matrix can reduce the original 4-fold symmetry of the tetragonal structure to the 2-fold symmetry of the orthorhombic structure. The DFT calculation results also indicate that the presence of Mg and Ag at the θ/α-Al interface can dramatically facilitate the θ formation. The clarification of the Ω structure and the role of Mg and Ag in θ formation provides insights into how to facilitate precipitation of an intrinsically strong equilibrium phase as a key strengthening constituent in the microstructure. Display omitted