A four-hole extension is also commonly used. The two middle holes are punched in accordance with ISO 838, and so paper punched with the four holes can be filed in binders that are consistent with ISO 838. The two additional holes are located 80 mm above and below these. The use of two additional holes provides more stability. This extension is sometimes referred to as the "888" system, because of the three 8-cm gaps between the holes. Some 2-hole punches have an "888" marking on their paper guide, to assist punching all four holes into A4 paper.