The major task of qualitative analysis of systems of ordinary differential equations is to recognize the global pattern of solution curves in the phase space. In this paper, I present a flow grammar, a grammatical specification of all possible patterns of solution curves one may see in the phase space. I describe flow pattern, a semi-symbolic representation of the pattern of solution curves in the phase space and show how an important class of flow patterns can be specified by the flow grammar. I then show that the flow grammar presented in this paper can generate any flow pattern resulting from any structurally stable flow on a plane. I also describe several properties of the flow grammar related to the enumeration of patterns. In particular, I estimate the upper limit of the number of application of rewriting rules needed to derive a given flow pattern. Finally, I briefly describe how the flow grammar is used in qualitative analysis to plan, monitor, and interpret the result of numerical computation.