In generating sentences by means of a transformational grammar, it is necessary to analyze trees, testing for the presence or absence of various structures. This analysis occurs at two stages in the generation process--during insertion of lexical items (more precisely, in testing contextual features), and during the transformation process, when individual transformations are being tested for applicability. In this paper the authors describe a formal system for the definition of tree structure of sentences. The system consists of a formal language for partial or complete definition of the tree structure of a sentence, plus an algorithm for comparison of such a definition with a tree. It represents a significant generalization of Chomsky's notion of "proper analysis," and is flexible enough to be used within any transformational grammar which the authors have seen. (Authors/AMM)