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You will never find an Olive Garden that is not within 3 miles of McDonald's. (And usually, it's less than 1 mile away.) In that case, there is a relationship between the two. Darden Restaurants, when selecting potential locations for a new Olive Garden, examines whether or not there is a successful McDonald's franchise nearby. If so, they contend that the market would be able to sustain an Olive Garden, too. They let McDonald's do the dirty work of site selection, in other words.
Is it a symbiotic relationship or a parasitic relationship? And how does that work for Wikipedia and porn? If there are people who bother to read pages about Medieval History, then many more will bother with accessing porn material over there?