Thick ropes last longer and usually hold more falls. Small diameter ropes are lighter and easier to clip into protection, making them the preferred choice for high-end lead climbing and glacier travel. Double ropes may be clipped alternately through the protection, reducing rope drag and decreasing the chances of rope failure over an edge. Both of the twin ropes are clipped through every protection point. The strands should never be separated.
Twin ropes are uncommon in the United States.
Double and twin ropes are designed to be used with a matching rope. Use of unmatched ropes will cause undue wear on one of the ropes, usually the one with the lowest working elongation or greatest diameter. Bi-colored ropes change colors or sheath patterns at the midpoint. This feature allows you to easily find the center of the rope. When in doubt, retire your rope.