We lay out a stylised accounting framework for system-wide debt capacity, when debt serves both as an obligation of the borrower, and also as the collateral pledged by the lender to secure additional funding. Our focus is on fluctuations in margins and leverage. Changes in margin (and the corresponding fluctuations in leverage) are reflected in the fluctuations in the balance sheet size of market participants and in the financial system's broader risk-taking capacity. A sharp increase in margins, especially after a protracted period of thin margins, will tighten financial conditions for the whole system. We use our framework to provide a perspective on the liquidity imbalances that rocked financial markets in March 2020, amid the uncertainty of the Covid-19 pandemic. […]