  It has been known for a while that the effective geometrical description of
compactified strings on $d$-dimensional target spaces implies a generalization
of geometry with a doubling of the sets of tangent space directions. This
generalized geometry involves an $O(d,d)$ pairing $\eta$ and an $O(2d)$
generalized metric $\mathcal{H}$. More recently it has been shown that in order
to include T-duality as an effective symmetry, the generalized geometry also
needs to carry a phase space structure or more generally a para-Hermitian
structure encoded into a skew-symmetric pairing $\omega$. The consistency of
string dynamics requires this geometry to satisfy a set of compatibility
relations that form what we call a Born geometry. In this work we prove an
analogue of the fundamental theorem of Riemannian geometry for Born geometry.
We show that there exists a unique connection which preserves the Born
structure $(\eta,\omega,\mathcal{H})$ and which is torsionless in a generalized
sense. This resolves a fundamental ambiguity that is present in the double
field theory formulation of effective string dynamics.
