When did nd become nn in which Early Irish environments?

This article examines the various accentual environments in which clusters of nasal plus voiced stop were assimilated to an unlenited nasal, focusing principally on nd > nn. It is argued that the chronological differentiation offered in GOI between (a) Early Old Irish nd > nn in proclitics and (b) Classical Old Irish nd > nn in stressed words is not sufficient. Instead, the accentual status of each syllable surrounding the cluster needs to be considered separately, and the chronological sequence needs to be enlarged to three stages: (a) Early Old Irish nd > nn between two pretonic syllables, (b) Classical Old Irish nd > nn between a stressed and a post-tonic or between two post-tonic syllables, and (c) Middle Irish nd > nn between a pretonic and a stressed syllable, occurring specifically in the article form ind and nasalised nd-. Some apparent exceptions to (c), suggesting pre-Middle Irish assimilation in this environment, are redefined as properly belonging to environment (a), and the appendix presents a complete sample, with statistical analysis, of relevant spellings (mainly of the article) from four texts of different dates of composition as preserved in the late Middle Irish manuscript Lebor na hUidre.