The following axioms are imposed on aggregations of choice functions: Unrestricted Domain (U), Neutrality (N), Independence of Symmetric Substitutions (ISS), Anonymity (A) and Positive Response (PR). Approval Voting is characterized by (U), (N), (ISS) and (PR); these axioms are independent. In the presence of (U), (N) and (PR), (ISS) is strictly stronger than (A). Plurality Voting is characterized by strengthening (U) along with (N), (ISS) and (PR).