The present dissertation, Part of speech — from Ancient Greece to Google: a study motivated by tagset conversion, is a linguistic study regarding gramatical word classes. This research is motivated by a specific task from Computational Linguistics: the annotation of part of speech (POS). Specifically, this dissertation reports the challenges and linguistic options arising from the task of aligning two tagsets: the first used in the annotation of the Mac-Morpho corpus — a Brazilian corpus with 1.1 million words — and the second proposed by Google research lab, which has been used in the context of the Universal Dependencies (UD) project. The present work adopts the annotation of large corpora as methodology and focuses mainly on the alignment of the past participle forms. As a result, in addition to the study and the documentation of the linguistic choices, this research provides a scenario which enables the study of the impact different tagsets have on Natural Language Processing (NLP) systems and presents another Portuguese NLP resource: the Mac-Morpho corpus annotated with project UD s tagset and consistent with its annotation philosophy, thus enabling future studies regarding the impact of different tagsets in the automatic processing of a language.