Let ${\cal M} =\langle M,<,\ldots\rangle$ bealinearly ordered structure. We define ${\cal M}$ to be{\em o-minimal} if every definable subset of $M$ is a finite union of intervals. Classical examples are ordered divisible abelian groups and real closed fields. Weprove a trichotomy theorem for the structure that an arbitraryo-minimal${\cal M}$ caninduce on aneighbourhood of any $a$in $M$. Roughly said, one of the following holds: \begin{enumerate}\item[(i)] $a$ istrivial (technical term), {\em or}\item[(ii)] $a$has a convex neighbourhood on which ${\cal M}$ induces thestructure of an ordered vector space, {\em or}\item[(iii)] $a$ is contained in an open interval on which ${\calM}$ induces the structure of an expansion of a real closed field. \end{enumerate} The proof uses ‘geometriccalculus’ which allows one to recover a differentiable structure by purely geometric methods.

1991Mathematics Subject Classification: primary 03C45; secondary 03C52, 12J15, 14P10.