Double Ropes

Half rope/double ropes are more for trad climbing than sport (although Pat at VE climbs sport with half ropes.) Good explanation here of what it is and how to do it.

“Double ropes should more correctly be referred to as half ropes. They are designed to be used as one of a pair - both ropes run alongside each other from the belayer to the climber (hence the term double rope technique, and hence why most belay plates have two slots in) and this dictates how the ropes are constructed and tested.

Don't confuse half-ropes with twin-ropes. Twin-ropes are even thinner than half-ropes and are designed to be used as a pair but when both ropes are to be clipped into every runner. We don't use them much in the UK as they intended for long mixed or ice lines where you want the simplicity of clipping both ropes together but the added security of the increases abrasion resistance and abseil potential of two ropes.”

 

Here is the section from Get Out On Rock explaining using double ropes.