We classify rank 2 cluster varieties (those for which the span of the rows of the exchange matrix is 2-dimensional) according to the deformation type of a generic fiber U of their X -spaces, as defined by Fock and Goncharov. Our approach is based on the work of Gross, Hacking, and Keel for cluster varieties and log Calabi-Yau surfaces. Call U positive if dim[Γ(U, O U )] = dim(U ) (which equals 2 in these rank 2 cases). This is the condition for the [GHK15b]-construction to produce an additive basis of theta functions on Γ(U, O U ). We find that U is positive and either finite-type or non-acyclic (in the usual cluster sense) if and only if the monodromy of the tropicalization U trop of U is one of Kodaira's monodromies. In these cases we prove uniqueness results about the log Calabi-Yau surfaces whose tropicalization is U trop . We also describe the action of the cluster modular group on U trop in the positive cases.