The linguistic quality of a parallel treebank depends crucially on the parallelism between the source and target language annotations. We propose a linguistic notion of translation units and a quantitative measure of parallelism for parallel dependency treebanks, and demonstrate how the proposed translation units and parallelism measure can be used to compute transfer rules, spot annotation errors, and compare different annotation schemes with respect to each other. The proposal is evaluated on the 100,000 word Copenhagen Danish-English Dependency Treebank.