Published online by Cambridge University Press: 25 January 2019
Given a weakly o-minimal structure  ${\cal M}$ and its o-minimal completion
${\cal M}$ and its o-minimal completion  $\bar{{\cal M}}$, we first associate to
$\bar{{\cal M}}$, we first associate to  $\bar{{\cal M}}$ a canonical language and then prove that Th
$\bar{{\cal M}}$ a canonical language and then prove that Th $\left( {\cal M} \right)$ determines
$\left( {\cal M} \right)$ determines  $Th\left( {\bar{{\cal M}}} \right)$. We then investigate the theory of the pair
$Th\left( {\bar{{\cal M}}} \right)$. We then investigate the theory of the pair  $\left( {\bar{{\cal M}},{\cal M}} \right)$ in the spirit of the theory of dense pairs of o-minimal structures, and prove, among other results, that it is near model complete, and every definable open subset of
$\left( {\bar{{\cal M}},{\cal M}} \right)$ in the spirit of the theory of dense pairs of o-minimal structures, and prove, among other results, that it is near model complete, and every definable open subset of  ${\bar{M}^n}$ is already definable in
${\bar{M}^n}$ is already definable in  $\bar{{\cal M}}$.
$\bar{{\cal M}}$.
We give an example of a weakly o-minimal structure interpreting  $\bar{{\cal M}}$ and show that it is not elementarily equivalent to any reduct of an o-minimal trace.
$\bar{{\cal M}}$ and show that it is not elementarily equivalent to any reduct of an o-minimal trace.