Ring binders are folders in which punched paper may be held by means of clamps running through the holes in the paper. The most common binder in the USA is a three ring system for letter size pages whereas most other countries use a two or four hole system for holding A4 sheets. The lever arch system is particularly useful for larger amounts of paper. The round ring style binder is easily the most common of all and is specifically designed for general use. Like the round ring style, the D-ring style also features three rings that unlock and lock to hold loose leaf sheets of paper. The only difference is the ring itself, which is shaped like the letter D and can usually accommodate 25% more sheets than the conventional round ring.