Stahl extended his proposals about collaboration theory during the next decade with his research on group cognition [http://GerryStahl.net/elibrary]. In his book on "Group Cognition", he provided a number of case studies of prototypes of collaboration technology, as well as a sample in-depth interaction analysis and several essays on theoretical issues related to re-conceptualizing cognition at the small-group unit of analysis. He then launched the Virtual Math Teams project at the Math Forum, which conducted more than 10 years of studies of students exploring mathematical topics collaboratively online. "Studying VMT" documented many issues of design, analysis and theory related to this project. The VMT later focused on supporting dynamic geometry by integrating a multi-user version of GeoGebra. All aspects of this phase of the VMT project were described in "Translating Euclid." Finally, "Constructing Dynamic Triangles Together" provided a detailed analysis of how a group of four girls learned about dynamic geometry by enacting a series of group practices during an eight-session longitudinal case study. The VMT project generated and analyzed data at the small-group unit of analysis, to substantiate and refine the theory of group cognition and to offer a model of design-based CSCL research.