  We present a detailed single crystal x-ray diffraction study of twinned
orthorhombic perovskites. The diffraction pattern can be indexed on a
$2a_p$x$2a_p$x$2a_p$ lattice, but does not obey cubic symmetry relations. The
data can be modelled in space group $Pnma$ with a twin based on a distribution
of the $b$ axis over three perpendicular cubic axes. This model allows full
structure determination in the presence of up to six twin fractions from single
crystal x-ray diffraction data.
