Macro-finance analyses commonly link firms’ borrowing constraints to the liquidation value of physical assets. For US nonfinancial firms, we show that 20% of debt by value is based on such assets (“asset-based lending” in creditor parlance), whereas 80% is based predominantly on cash flows from firms’ operations (“cash flow–based lending”). A standard borrowing constraint restricts total debt as a function of cash flows measured using operating earnings (“earnings-based borrowing constraints”). These features shape firm outcomes on the margin: first, cash flows in the form of operating earnings can directly relax borrowing constraints; second, firms are less vulnerable to collateral damage from asset price declines, and fire sale amplification may be mitigated. Taken together, our findings point to new venues for modeling firms’ borrowing constraints in macro-finance studies.