This paper deals with some theoretical aspects of how to write a contrastive grammar. Generally, non-diachronic comparative research on grammar can be divided into two sub-disciplines: contrastive linguistics and general-comparative, i.e. typological linguistics. Although differing in perspective, method and research aims, both sub-disciplines share the problem of how to define appropriate tertia comparationis. This becomes even more difficult when writing a comprehensive comparative grammar for two or more languages. This paper discusses various angles from which such a contrastive grammar can be organized and shows which tertia comparationis result from which of the different viewpoints. The specific effects on contrastive grammar writing are illustrated by contrasting data from German and Estonian. Finally, the paper argues for taking a functional-semantic approach to comparative grammar writing.