What? A triangle?
It's a very simple fold that creates a very practical item -- it's a bookmark that slips over the corner of the page -- no damage to the spine of the book, just a slim page keeper.
Modeling the bookmark: Women's Work the First 20,000 Years: Women, Cloth, and Society in Early Times by Elizabeth Wayland Barber. It's a rather academic tome, but a fascinating look at the history of cloth and fiberwork -- a special interest of mine :).