Letter of Guarantee

A letter from a bank to a brokerage firm which states that a customer (who has written a call option) does indeed own the underlying stock and the bank will guarantee delivery if the call is assigned. Thus the call can be considered covered. Not all brokerage firms accept letters of guarantee. Also: letter issued to Option Clearing Corporation by member firms covering a guarantee of any trades made by one of its customers, (a trader or broker on the exchange floor).