Asset-backed securities for which the underlying asset portfolios are debt obligations: either bonds (collateralised bond obligations) or loans (collateralised loan obligations) or both. The credit exposure underlying synthetic CDOs derives from credit default swaps. The CDOs issued by an individual vehicle are usually divided in different tranches: senior tranches (rated AAA), mezzanine tranches (AA to BB), and equity tranches (unrated). Losses are borne first by the equity securities, next by the junior securities, and finally by the senior securities; junior tranches offer higher coupons (interest payments) to compensate for their increased risk. Collateralised debt obligation squared (CDO-squared) is a type of collateralised debt obligation where the underlying asset portfolio includes tranches of other CDOs.