  In the context of representation theory of finite dimensional algebras,
string algebras have been extensively studied and most aspects of their
representation theory are well-understood. One exception to this is the
classification of extensions between indecomposable modules. In this paper we
explicitly describe such extensions for a class of string algebras, namely
gentle algebras associated to surface triangulations. These algebras arise as
Jacobian algebras of unpunctured surfaces. We relate the extension spaces of
indecomposable modules to crossings of arcs in the surface and give explicit
bases of the extension spaces for indecomposable modules in almost all cases.
We show that the dimensions of these extension spaces are given in terms of
crossing arcs in the surface.
  Our approach is new and consists of interpreting snake graphs as
indecomposable modules. In order to show that our basis is a spanning set, we
need to work in the associated cluster category where we explicitly calculate
the middle terms of extensions and give bases of their extension spaces. We
note that not all extensions in the cluster category give rise to extensions
for the Jacobian algebra.
