WDYW.01: Tone Restoration for Standard Yorùbá

Purpose: This paper addresses automatic tone restoration for Standard Yorùbá, a three-tone language whose digital texts are predominantly written without tone marks. It presents ToneMarker, the first module (WDYW.01) of the Well-formed Digital Yorùbá Writing (WDYW) research programme, and evaluates its performance on an independent test set. Design/methodology/approach: ToneMarker is a freely accessible, browser-based tool built on a verified corpus of 11.8 million words. It employs a four-layer architecture combining expert-verified corrections, sentence-level matching, context-based statistical modelling, and frequency-based prediction, augmented by a grammar-based disambiguation layer for selected high-frequency ambiguities. Performance was evaluated on a gold-standard test set of 48 Yorùbá Wikipedia sentences independently tone-marked by the author. Findings: ToneMarker achieved 77.1% word-level accuracy (95% CI: 74.1–79.8%), increasing to 78.3% when the grammatically irreducible ni/ní homophony was excluded. The study also identifies and resolves a previously under-documented double-diacritic-stripping bug that silently corrupts Yorùbá text in standard tokenisation pipelines and demonstrates that Unicode normalisation is essential for accurate evaluation of tone-restoration systems. Originality: Within the author’s knowledge, this is the first Yorùbá tone-restoration system to combine corpus-scale statistical modelling with a grammar-based disambiguation layer. It is also, as far as we are aware, the first study to document and quantify the effects of double-diacritic stripping in Yorùbá NLP pipelines. Contribution to the field: The paper provides a freely deployable tool for Yorùbá scholars, students, and language professionals while establishing a foundation for future lexical, grammatical, and neural approaches to digital Yorùbá writing support.