The primary concern of the thirty eight files in this series is tracing inventory of paintings owned or commissioned by aristocratic families in Great Britain.
File titles contain a mixture of family surname, peerage and seat/house name. Millar's interest seems to have been spread across all aspects. Therefore where known the full family name, peerage title and location of seat/house contained within the files are all recorded in the description.
There is evidence of visits to local record offices, country houses and correspondence with heirs and fellow academics.
Several files contain research on specific topics/themes to be written about, often later published as articles in journals such as the Burlington Magazine. Where known the bibliographical details of the publication are recorded in the description.