  Using computer simulations, we demonstrate a new type of commensurability
that occurs for vortices moving longitudinally through periodic pinning arrays
in the presence of an additional transverse driving force. As a function of
vortex density, there is a series of broad maxima in the transverse critical
depinning force that do not fall at the matching fields where the number of
vortices equals an integer multiple of the number of pinning sites. The
commensurability effects are associated with dynamical states in which evenly
spaced structures consisting of one or more moving rows of vortices form
between rows of pinning sites. Remarkably, the critical transverse depinning
force can be more than an order of magnitude larger than the longitudinal
depinning force.
