The national debt is the sum of money that the federal government has borrowed to cover the outstanding balance of expenses incurred over time. In a given fiscal year (FY), when expenses (e.g., money for roads) exceed revenues (e.g., money from federal income tax), a budget deficit results. To pay for this deficit, the federal government borrows money by selling negotiable securities, such as treasury bonds, bills, notes, variable rate notes, and treasury inflation-protected securities (TIPS).