learning about sidewalls

We visited the newly reopened Cooper-Hewitt Museum when I was in New York City for a couple days between Christmas and New Year's. I kept seeing the term "sidewalls" and was unfamiliar with the term for what looked to me like wallpaper. Checking the Art & Architecture Thesaurus, I discovered that AAT uses the term "fill papers" for "Wallpapers used to cover the main portions of walls between borders." "Sidewalls" is one of the alternate terms. Who knew? Saying "wallpaper" is just way too ambiguous.

This is an Aesthetic or Japanesque-Style wall paper, ca. 1880, from the Cooper-Hewitt collection.