Mackinnon appears to have written some notes, perhaps in preparation to publish a textbook that he had possibly expected to title 'Celtic Philology'. This is the first part, which he has titled 'Phonology'. Despite this title, much of the notebook's contents are descriptions, as they were understood at the time, of the classification of Celtic languages as a branch of Indo-European, how they are related to other languages, and a detailed account of the linguistic typology of Celtic languages. The second half of the notebook seems to be a dictionary of phonological terms, specific to Celtic languages, and examples principally from Goidelic languages.