There are 3 types of Dynamic climbing ropes and knowing how and when to use them is crucial to safety and to efficiency when at the crags. The word Dynamic refers to them having the ability to stretch and absorb the impact of a falling climber. Let us first of all understand the basic properties of each type of rope.
Single or Full-rope is the term used to indicate that it has the full required strength of a rock climbing/ mountaineering rope, and this is shown at the end of the rope buy a figure 1 in a circle at either end. These are typically the ropes sport climbers use, indoors and outdoors and in some instances in Trad climbing. They can also be used in mountaineering situations, such as moving together over easy terrain or glacier travel. In all these situations a lead climber would tie in to a single strand of the rope.
Double or Half-ropes have about 2/3 the required strength of a Single rope and carry a figure ½ in a circle at either end. These ropes are typically used in Trad climbs and sometimes on multi-pitch sport climbs, when long abseils are going to be encountered or anytime when there are 3 members in a party. The rule here is that the lead climber must always tie into two strands of Half-rope. The people seconding can safely use single strands to follow the climb. Half- ropes are also used when climbing ice ropes. Note that it is best to keep the impact force as low as possible on protection points (ie running belays), therefore when Trad climbing do not clip both Half-ropes into the same running belay. It is permissible to clip the two ropes through the same running belay on traverse pitches, when it is technically impossible for high impact forces to occur. The rule is however, that if one running belay point has both ropes placed in the same karabiner, then the ropes must be placed together in all protection on that pitch. Putting some running belays with both ropes and others alternatively can result in ropes burning against each other in a leader fall.
Twin-Ropes are indicated by two inter-locked circles on the ends of the rope. Twin ropes are two thin ropes which, when leading on, are used as if they were a Single rope. When used like this they have similar properties as a Single rope. They are usually used on long alpine routes and ice climbing and are very seldom used in South Africa.
There is actually a fourth type of rope, namely a Walking-rope. This is in fact not a separate rope designation, but simply a single strand of Twin-rope used only in low impact situations, such as scrambling and short-roping. These ropes carry the Twin-rope insignia but are used in non-serious situations when hiking and scrambling.