Ezquerro, AnaVilares, DavidYli-Jyrä, AnssiGómez-Rodríguez, Carlos2025-09-112025-09-112025-07Ana Ezquerro, David Vilares, Anssi Yli-Jyrä, and Carlos Gómez-Rodríguez. 2025. Hierarchical Bracketing Encodings for Dependency Parsing as Tagging. In Proceedings of the 63rd Annual Meeting of the Association for Computational Linguistics (Volume 1: Long Papers), pages 18436–18450, Vienna, Austria. Association for Computational Linguistics. DOI: 10.18653/v1/2025.acl-long.903979-8-89176-251-0https://hdl.handle.net/2183/45744Presented at the 63rd Annual Meeting of the Association for Computational Linguistics, July 2025, Vienna, Austria.[Abstract]: We present a family of encodings for sequence labeling dependency parsing, based on the concept of hierarchical bracketing. We show that the existing 4-bit projective encoding belongs to this family, but it is suboptimal in the number of labels used to encode a tree. We derive an optimal hierarchical bracketing, which minimizes the number of symbols used and encodes projective trees using only 12 distinct labels (vs. 16 for the 4-bit encoding). We also extend optimal hierarchical bracketing to support arbitrary non-projectivity in a more compact way than previous encodings. Our new encodings yield competitive accuracy on a diverse set of treebanks.engAttribution 4.0 Internationalhttp://creativecommons.org/licenses/by/4.0/Dependency parsingTaggingEncodingsconference outputopen access10.18653/v1/2025.acl-long.903