The files in this series contain material on subjects and themes in XVIth and XVIIth century art. They were collected and compiled by Millar throughout his career, alongside the artist research files (see ONM/2) and relate mainly to styles and eras, e.g. miniatures, equestrian, topographical, sculpture, Frenchified, Jacobean, Late Stuart, Early XVIth century), and specific collections (Dartmouth, Tyninghame, Ickworth).
The file titles are in inverted commas and are direct quotes from Millar's original slip files.
As with the artist files, they are primarily a visual resource containing a large number of black and white photographs and images of paintings. They also include correspondence with owners, auction houses, academics, museums and galleries worldwide; research notes; journal articles; summary descriptions of paintings; exhibition catalogues and pamphlets.
Most files contain a number of images of works relating to the stated subject. The total number of images is listed in the 'extent' field. The file description does not include any additional information about individual images, but each item of non-image material (ie. correspondence, notes etc.) is listed in detail.
The research files were kept by Millar at his home before transferral to the Paul Mellon Centre in 2008. They were viewed by a number of researchers prior to cataloguing but have not been merged or re-arranged by Archivists, other than to alphabeticise the file order. The content of each file has been re-packaged into archival folders from the original aging slip files. It is clear in some cases that Millar intended to re-arrange certain files at a later date, e.g. ONM/3/3, 'Dutch: Miscellaneous unsorted'; ONM/3/40, 'Unimportant but check'; ONM/3/41, 'Miscellaneous: 1660-1685 to be sorted'.