  Soft-collinear effective theory is generalized to include soft massless
quarks in addition to collinear fields. This extension is necessary for the
treatment of interactions with the soft spectator quark in a heavy meson. The
power counting of the relevant fields and the construction of the effective
Lagrangian are discussed at leading order in Lambda/m_b. Several novel effects
occur in the matching of full-theory amplitudes onto effective-theory operators
containing soft light quarks, such as the appearance of an intermediate mass
scale and large non-localities of operators on scales of order 1/Lambda.
Important examples of effective-theory operators with soft light quarks are
studied and their renormalization properties explored. The formalism presented
here forms the basis for a systematic analysis of factorization and power
corrections for any exclusive B-meson decay into light particles.
