The way I understand it there is no difference between a double and a half rope. When you use two half ropes together it is a double rope. Twins are another type of double rope system but when most people talk about using double ropes they are usally talking about using 2 half ropes. There are two primary differences. The first is how you use them, alternating gear vs. clipping both ropes into the same piece. The second are the specs. Half ropes usally fall into the mid 8mm range, where twins range from mid 7mm to low 8mm. Half ropes and twins are tested differently, so if your going out to buy new ropes keep that in mind. The specs between half ropes and twins are usually very differnt.
