In side-by-side programming, two programmers (typically working on related aspects of a project) move their computers so close to one another that they can eﬀortlessly change between working alone and working together, where working alone is the primary mode. The technique was proposed in order to obtain some of the advantages of pair programming at much lower overhead. As a ﬁrst step towards understanding how and when to use side-by-side programming, the present study aims at describing when and for what purpose side-by-side programmers get together to cooperate. The main result is a classiﬁcation of the cooperation episodes by purpose and content into diﬀerent types: Exchange project details, Exchange general knowledge, Discuss strategy, Discuss step, Debug work product, Integrate work products, and Make remark. These types were derived via the Grounded Theory method and are described conceptually in terms of the types of events of which they consist. All concepts used in these descriptions are grounded in actual observations.