Toulmin's argumentation model is an effective tool in the description of one step of an argument. However, there is a need to consider more than this model to portray a comprehensive argumentation process. The purpose of the study is to investigate types of global argumentation structures that emerged while prospective middle school mathematics teachers were working on conjecture-generation activities related to geometry. The participants were junior prospective middle school mathematics teachers at a state university in Ankara, Turkey. The eight global argumentation structures that emerged via conjecture-generation activities of two groups were classified as mono structures and hybrid structures. According to the findings, three types of mono structures, which are the reservoir-structure, the funneling-structure, and the branching-structure, were seen among four possible cases. Three types of hybrid structures were observed among eleven possible combinations of the mono structures, namely, the reservoir-funneling-structure, the line-branching-structure, and the line-reservoir-branching-structure.