Swap exchanging interest and principal cash flows denominated in two different currencies; unlike a single-currency IRS, the principals are exchanged at inception and re-exchanged at maturity at the initial spot. Most commonly traded as a basis swap: floating-vs-floating (e.g. SOFR vs ESTR) with a cross-currency basis spread, s_ccy, that has been persistently non-zero since 2008 (covered-interest-parity violation), reflecting USD funding scarcity and balance-sheet costs. Used by issuers to swap a non-USD bond into a synthetic USD floater.