The current Japanese writing system is characterised by its mixed and complementary use of four main scripts called kanji, hiragana, katakana and rōmaji. Regarding rōmaji, or ‘Roman script’, it is commonly described as ‘alphabetic’, ‘phonemic’ or ‘segmental’. All these descriptors imply that the script operates primarily on a systematic mapping between graphs and individual phonemic segments. However, while rōmaji actually implements monosegmental mappings in romanisation (e.g. <sushi>: <s> → /s/, <u> → /ɯ/, <sh> → /ɕ/, <i> → /i/), it has many other uses that are often overlooked. This study distinguishes six categories (Coulmas 1996; López Rúa 2004; Irwin 2011): 1. Acronym: <Suica> suika ‘Super Urban Intelligent Card’ (prepaid travel card) 2. Alphabetism: <NHK> enueichikē ‘Nippon Hōsō Kyōkai’ (Japan Broadcasting Corporation) 3. Letter-name word: <L> in <L字型> erujigata ‘L-shape’ 4. Abbreviation: <GW> gōrudenwīku ‘Golden Week holidays’ 5. Xenography: <WC> toire ‘toilet’ (< Eng. water closet) 6. Loan orthography: <TikTok> tikkutokku ‘TikTok’ Of these, only Type 1 is monosegmental in the strict sense (e.g. <s> → /s/). Types 2 and 3 are polysegmental because all rōmaji letter names comprise multiple phonemes in Japanese (e.g. <N> → /enɯ/). Types 4 and 5 are morphographic because the graphs map to individual morphemes (e.g. <G> → {gōruden}). Type 6 is in a grey zone, where some mappings are monosegmental (e.g. <T> → /t/) but others are polysegmental due to phonotactic restrictions in Japanese (e.g. <k> → /kɯ/). Unlike in writing systems based principally on the Roman script (e.g. English), polysegmental and morphographic mappings are not exceptional for rōmaji usage within the multi-script Japanese writing system. The present paper demonstrates this point based on a survey of the Balanced Corpus of Contemporary Written Japanese. It further discusses the significance of non-segmental adaptations of segmental scripts for the description and classification of writing systems. Coulmas, Florian. 1996. The Blackwell Encyclopedia of Writing Systems. Oxford & Malden: Blackwell Publishers. Irwin, Mark. 2011. Loanwords in Japanese. Amsterdam & Philadelphia: John Benjamins Publishing Company. López Rúa, Paula. 2004. Acronyms & Co.: A typology of typologies. Estudios Ingleses de la Universidad Complutense 12. 109-129.