Historical linguistics, whether synchronic or diachronic, is by definition based on corpora.Since we do not have access to the intuitions of native speakers we can only test linguistic hypotheses about historical languages by systematically collating information from our corpus of texts.For questions that typically concern linguists, this often means identifying every occurrence of a particular phenomenon in the corpus, analysing, classifying and counting the occurrences and then using this for testing hypotheses about the structure of the language.This can be done manually, but this is time-consuming and error-prone.As Haug (2015) points out, while reading the text and manually collating information from it is essential for hypothesis formation it is much less useful for hypothesis testing.Even if the text is in electronic form, it is easy to overlook an example, record it incorrectly or fail to apply test criteria consistently over time.This paper focuses on treebanks, which are corpora that have been annotated with morphosyntactic information so that we can extract linguistic structures like 'verb with an accusative noun'.High-quality treebanks for a range of historical languages now exist and are widely used in historical linguistic research.This includes treebanks that follow the Penn-style of annotation, e.g. the Penn-Helsinki