General purpose debit card purchase volume in the U.S. is dominated by Visa and MasterCard. Combined, they hold a 79% share. Discover holds less than a 1% share. Credit card brands dominate because debit card payments at merchants in the U.S. are overwhelmingly secured by signatures, not PINs. Signature-based debit payments, like credit card payments, are cleared and settled in two separate data messages. The authorization message is subsequently followed by the settlement information message. PIN-based debit payments are authorized and settled in a single transaction. Star is largest among...