A classic bone folder is made of real bone, not plastic or Teflon, and resembles a fat, blunt-edged tongue depressor, rounded at one end and pointed at the other. With it, I can turn a digital print, piece of cardstock or watercolor paper into a professional-looking note or greeting card.
Pulling the pointed end alongside a straight edge and across the paper produces a subtle score that facilitates a perfect fold. Next, I fold the card very gently by hand along the score, and then stroke one of the short, straight sides of the bone folder along the score to flatten the rounded fold to a sharp crease.
When sending a letter that I want to look good, I make two quick strokes of the folder along preliminary hand folds to create folded edges that are sharp and square. Bone folders also can be used to burnish paper as it is glued to cardstock, album or scrapbook pages. They produce accurate and sharp folds and creases on origami papers as well as facilitate sculpting, architectural modeling or bookbinding with paper.
I recommend rubbing your bone folder with olive oil from time to time to avoid flaking or brittleness. Folders made of real bone are best, unless you wish to use a Teflon folder to avoid the slight luster sometimes created by the friction of a real bone folder.
[I relied on one of these while producing a large batch of homemade invitations and can attest to its utility. Here’s a look at a bone folder in use. -- ES]