The Eurosystem requires that a specified margin be maintained over time on the underlying assets used in its liquidity-providing reverse transactions. This implies that if the regularly measured market value of the underlying assets falls below a certain level, counterparties have to supply additional assets (or cash). Similarly, if the market value of the underlying assets, following their revaluation, were to exceed the amount owed by a counterparty plus the variation margin, the central bank would return excess assets (or cash) to the counterparty.