they differentiate the products in question from other products to such an extent that they are only to a small degree interchangeable with such other products and therefore not effectively constrained by them at competitive prices.19 In making this assessment regard must in particular be had to the needs of marginal consumers. In most cases it is not decisive that a certain group of consumers does not consider the products in question to be good substitutes. What matters is that a sufficiently large number of consumers do consider that a product is a good substitute for the product supplied by the undertaking concerned. If so, the two products form part of the same market, unless a single supplier of the product in question would be able to sell to consumers with less elastic demand at a higher price and prevent consumers with more elastic demand from reselling to the former group of consumers. In that case the group of consumers with less elastic demand may be in a market of their own.20
Geographic Market 19. It may also be relevant to compare prices across various regions. This may in the first place be helpful to determine the geographic market, but may also be useful in determining the product market. If an undertaking supplies a product in several regions and charges higher prices in regions where it has a higher share of sales of that type of product, it is an indication that the main competitive constraint comes from other suppliers of that type of product and not from suppliers of other types of products. Even if the undertaking in question does not itself supply its product in different regions, a similar analysis may be possible if the same type of product is sold in other regions by other undertakings. Price comparisons across regions should, however, take into account whether there are other factors differing between the regions than the intensity of competition.
See in this respect Case T-86/95, Compagnie générale maritime and Others v Commission  ECR-1011, paragraph 48, and Case 27/76 United Brands, cited in footnote 5, paragraph 12, Case 66/86, Ahmed Saeed Flugreisen and Silver Line Reisebüro GmbH v Zentrale zur Bekämpfung unlauteren Wettbewerbs e.V  ECR 803, paragraphs 39 and 40. See paragraph 43 of the Commission Notice on the definition of the relevant market cited in footnote 11.