  I propose the use of CP-odd invariants, which are independent of basis and
valid for any choice of CP transformation, as a powerful approach to study CP
in the presence of flavour symmetries. As examples of the approach I focus on
Lagrangians invariant under $\Delta(27)$. I comment on the consequences of
adding a specific CP symmetry to a Lagrangian and distinguish cases where
several $\Delta(27)$ singlets are present depending on how they couple to the
triplets. One of the examples included is a very simple toy model with explicit
CP violation with calculable phases, which is referred to as explicit
geometrical CP violation by comparison with previously known cases of
(spontaneous) geometrical CP violation.
