  We study B-type D-branes in linear sigma models with Abelian gauge groups.
The most important finding is the grade restriction rule. It classifies
representations of the gauge group on the Chan-Paton factor, which can be used
to define a family of D-branes over a region of the K\"ahler moduli space that
connects special points of different character. As an application, we find a
precise, transparent relation between D-branes in various geometric phases as
well as free orbifold and Landau-Ginzburg points. The result reproduces and
unifies many of the earlier mathematical results on equivalences of D-brane
categories, including the McKay correspondence and Orlov's construction.
