  Committee scoring rules form a rich class of aggregators of voters'
preferences for the purpose of selecting subsets of objects with desired
properties, e.g., a shortlist of candidates for an interview, a representative
collective body such as a parliament, or a set of locations for a set of public
facilities. In the spirit of celebrated Young's characterization result that
axiomatizes single-winner scoring rules, we provide an axiomatic
characterization of multiwinner committee scoring rules. We show that committee
scoring rules---despite forming a remarkably general class of rules---are
characterized by the set of four standard axioms, anonymity, neutrality,
consistency and continuity, and by one axiom specific to multiwinner rules
which we call committee dominance. In the course of our proof, we develop
several new notions and techniques. In particular, we introduce and
axiomatically characterize multiwinner decision scoring rules, a class of rules
that broadly generalizes the well-known majority relation.
