let's develop on this simple bank example assume a brand-new bank with an empty balance sheet, no revenue, no deposits, no cashflow and no regulations the bank is about to finance its first asset - a UST bond with a liability - bonds issued by the bank
this newly acquired UST bond was financed with some liability of the bank let's say the bank itself issued bonds with a smaller coupon than USTβs thus, the bank used a liability to finance and asset and earns a spread