Synthetic bone folder, available in two sizes.
The smaller rounded tool measures 12cm X 2.5cm and is good for neatly folding and creasing cards, paper, and book cloth in hard back covers. 
The larger tool measures 21cm X 2.5cm and is good for burnishing, embossing, and flattening glued/taped covers to ensure adhesion and removal of air bubbles.
If you're stocking up, both sizes of bone folder come as part of the Bookmaking Essentials Tool Pouch, so it may be worth checking this out.
Top tip: Always fold your papers as a group when creating signatures, rather than one at a time. This creates a tight and nestled rounded spine.