Ring binders are folders in which punched pieces of paper may be held by means of clamps or rods running through holes in the paper. These retainers are usually spring-loaded, frequently but not invariably circular, and may or may not have additional latching systems. Binders come in many standard sizes with respect to both capacity and paper size. The most common type in the United States is a three ring system for letter size pages (8½ × 11), whereas most other countries use a two or four hole system for holding A4 sheets. A six hole system is avialble to support the filoFAX range.