(Note: both VViews and JKP have written a lot about this subject, and there are also the Sozomeno drawings Rene posted, so see this as an addition rather than something completely new).
While browsing the Très Riches Heures illustrations (You are not allowed to view links.
Register or
Login to view.) I noticed that the main drawings on certain pages had been framed with architectural elements. Pillars with statues on pedestals, sometimes wooden bows flanked by two figures spanning the top.
Examples:
You are not allowed to view links.
Register or
Login to view.
You are not allowed to view links.
Register or
Login to view.
You are not allowed to view links.
Register or
Login to view.
You are not allowed to view links.
Register or
Login to view.
You are not allowed to view links.
Register or
Login to view.
You are not allowed to view links.
Register or
Login to view.
You are not allowed to view links.
Register or
Login to view.
The role of these figures appears to be that of "thematically appropriate onlooker", for example with Job there are living Dead, with David armored knights. There often is a certain hierarchy in the figures' vertical position, with heavenly beings at the top (cfr. VM cloud band figures).
These rich frames are typical for books of hours of the first half of the 15th century. A turning point came in the 1450's with Jean Fouquet, who considered borders a waste of space and treated each manuscript page as a painter would his canvas. You are not allowed to view links.
Register or
Login to view.
----
The nymphs in Q13A (using Claston's term) have some similarities with these architectural frames. They each stand like a statue on a base - free-floating nymphs are very exceptional). Some pages use the motif of the wooden beam on top. Some pages use pillars to connect them vertically, though the water makes them more like tubes. My personal impression is that the VM illustrator(s) took the main illustrations and tucked them away in what looks like an architectural frame. While they clearly allude to a style less rich than that of the Très Riches Heures, it is possible that they were imitating this manner of framing.
For me though (and this is where disagreement might arise), they really appear as illustrations in their own right, creating some tension between layout and contents.