Drawing on the archival study of over 1,600 copies of the Tibetan Aparimitāyur-nāma mahāyāna-sūtra (Tib.: Tshe dpag du myed pa'i mdo) produced in Dunhuang from the 820s to the 840s and now kept in the British Library, this article sheds light on the orthographic norms of Middle Old Tibetan writing. Based on editors' corrections, and on a corpus of nearly 200 transcribed explicits, the article compares the orthographic norms of this group of sutras with those of other dated Old Tibetan manuscripts and inscriptions. It proposes that among the most important markers for dating Old Tibetan writing are the increased use in the tenth century of the ’i(s) form of the genitive and ergative particles as a separate syllable, and the relative absence of the tu form of the terminative particle in Middle Old Tibetan writing. Additionally, the study offers suggestions concerning the development of the various forms of Tibetan case particles.
