  We give a definition of a what we call a `tonnetz' on a triangulated surface,
generalising the famous tonnetz of Euler from 1739. In Euler's tonnetz the
vertices of a regular `$A_2$ triangulation' of the plane are labelled with
notes, or pitch-classes. In our generalisation we allow much more general
labellings of triangulated surfaces. In particular, edge labellings turn out to
lead to a rich set of examples. We construct natural examples that are related
to crystallographic reflection groups and live on triangulations of tori.
Underlying these we observe a curious relationship between mathematical
Langlands duality and major/minor duality. We also construct `exotic'
type-$A_2$ examples (different from Euler's Tonnetz), and a tonnetz on a sphere
that encodes all major ninth chords.
