We consider the design of an ontology for marketing knowledge. Such an ontology contains two hierarchies, a customer hierarchy and a product hierarchy. The product hierarchy representation is straight-forward, as in general each level consists of products that are more specific than the products on the previous level. However, the customer hierarchy is problematic, since it involves many independent dimensions such as age, gender, income, etc. A straightforward ordering of the different dimensions to create a tree hierarchy is ineffective. We present an innovative design for the customer hierarchy based on introducing intersections of options for various dimensions on demand. We call such an ontology an intersection ontology. The advantages of such a design are explored and evaluated using our Web marketing project.