Due to the difficulty of calling all customers whose margin accounts have fallen in value for the day, a clearing member firm usually will require that a sum of money be deposited at the initiation of any futures position. This additional sum is called a maintenance margin. In most situations, the original margin requirement may be established with a risk-free, interest-bearing security such as a T-bill. However, the maintenance margin, which must be in cash, is adjusted for daily changes in the contract value.