Tthe back side panels represent the Annunciation, painted in grisaille. See the link provided.
panels
It is called a triptych. They were common in middle ages religious art. Two panels is called a diptych.
A triptych is a painting made in three sections that fit together. It can also be three relief carved panels.
tetraptych
A triptych. Split into two is a diptych. The name applies where a single image extends across all panels, or separate images on each panel.
panels
panels
It is called a triptych. They were common in middle ages religious art. Two panels is called a diptych.
Panels (apex)
The total width of the triptych being 45 inches, each of the thre panels has a width of 15 inches.
A triptych is a painting made in three sections that fit together. It can also be three relief carved panels.
p=63/3
P=73/3
tetraptych
A triptych. Split into two is a diptych. The name applies where a single image extends across all panels, or separate images on each panel.
There is no synonym for diptych. However, if the work of art were to have three panels, it would be known as a "triptych," a "tetraptych" if there are four, and so on.
A panel is generally a unit that makes up a multi-piece painting. If the painting has two panels, it is called a diptych. Three panels would make it a triptych. One of the reasons they may be called panels is that they may predate the popular use of stretched canvas and be painted upon wood panels.