Bone folders are very handy when creasing, burnishing (flattening) and gluing paper or any number of other materials. Originally made of bone, book binders and paper artists now prefer teflon.
Teflon has a couple magical properties that make it ideal for use with paper. Most importantly, it is non-stick so glue, ink, paint, etc... won't stick and build up over time. Second, it does not damage the paper or leave a mark when folding, burnishing or scoring (very useful when you're working with fine papers).
Each bone folder is 5-3/4" x 5/8" x 1/4" (14.6cm long).