A cash-settled index future contract worked and, some details aside, still works as follows. (I draw my example from Martin 1988, pp. 139-140.) Suppose the S&P 500 index level was 285.00, and a future on the index, maturing in three months, can be bought at 286.40. The "contract multiplier" in the 1980s was $500, so a single contract had a notional value of the index level (285) multiplied by $500, or $142,500. The purchaser of such a contract, however, had to deposit with the Mercantile Exchange's clearinghouse only an "initial margin" of $10,000 (which could, for example, be in the form of U.S. Treasury bills, so one does not have to forgo interest on the margin payment). The seller also had to deposit a similar margin.

Donald MacKenzie, An Engine, Not a Camera, loc. 775