  We present a unifying description for the martensitic transformation of steel
that accounts for important experimentally observable features of the
transformation namely, the Neumann bands, the interfacial (habit) plane between
the transformed and untransformed phases and their orientation relationship
(OR). It is obtained through a simple geometric minimization of the total
distance traveled by all the atoms from the austenite (FCC or $\gamma$) phase
to the martensite (BCC or $\alpha$) phase, without the need for any explicit
energy minimization. Our description unites previously proposed mechanisms but
it does not rely on assumptions and experimental knowledge regarding the shear
planes and directions, or external adjustable parameters. We show how the
Kurdjumov-Sach orientation relationship between the two phases and the
$\{225\}_\gamma$ habit plane, which have both been extensively reported in
experiments, naturally emerge from the distance minimization. We also propose
an explanation for the occurrence of a different orientation relationship
(Pitsch) in thin films.
