  \theta' (Al2Cu) precipitates in Al-Cu-Ag alloys were examined using high
angle annular dark field scanning transmission electron microscopy
(HAADF-STEM). The precipitates nucleated on dislocation loops on which
assemblies of {\gamma}' (AlAg2) precipitates were present. These dislocation
loops were enriched in silver prior to \theta' precipitation. Coherent, planar
interfaces between the aluminium matrix and \theta' precipitates were decorated
by a layer of silver of two atomic layers in thickness. It is proposed that
this layer lowers the chemical component of the Al-\theta' interfacial energy.
The lateral growth of the \theta' precipitates was accompanied by the extension
of this silver bi-layer, resulting in the loss of silver from neighbouring
\gamma' precipitates and contributing to the deterioration of the \gamma'
precipitate assemblies.
