Design choices for collaborative tools are often made on a technical or arbitrary basis. In some cases, such choices have a pedagogical justification; we posit that a design rationale appealing to theoretical concepts in cognitive and pedagogical sciences is sounder than one that makes consensus choices based on availability of technology. We explain how a cognitive model of cooperation helped shape our decisions for the design of a collaborative text editor. We established links between alternative designs and the forms of cooperation that were thus rendered possible (as defined by this model). As research questions were centered on the forms of cooperation which were likely to emerge, we were eager to minimize the constraints imposed by the tool. Our view is that there is a strong relationship between cognitive models of interaction and the collaborative properties of CSCL tools that can be wisely exploited when designing such tools.