A form of long term cargo insurance contract. It has no aggregate limit but, subject to a limit to the amount at risk in any one vessel, and often a limit to the amount at risk in any one location prior to shipment, the contract covers all shipments forwarded by the assured during the currency of the open cover. Underwriters have the right to cancel at any time by giving the requisite notice of their intention to cancel, but shipments that have commenced transit before the notice period expires continue to be covered until final delivery within the terms of the transit clause.