A relatively unknown form of advertising, co-op is a cost-sharing arrangement in which manufacturers and suppliers provide financial assistance for customers' advertising programs. It can often make the difference in whether or not a small business can afford to advertise at all. Co-op advertising works in a number of ways. For example, a retailer can buy goods from a manufacturer and accrue co-op funds based on the amount of purchase. Those funds can be returned in the form of a cash rebate, a credit to the retailer's account, or in products. In case you're thinking your firm is too specialized to take advantage of co-op advertising, you may want to reconsider. The Co-op Handbook, published twice a year by the Yellow Pages Publishers Association (YPPA), contains thousands upon thousands of co-op program listings. Indexed by brand name and company, each listing provides the company name, the type of plan it offers, whether mention of competitors is allowed, and the type and size of ads required.