Its function is to unite the climbing anchors with the rope of the climber so that the rope passes freely through one of the carabiners and the other one remains attached to the anchorage. Its function is to make a joining point and allow a little distance so that the climber can move more easily and easily. It also makes rope friction smaller and that the rope movement affects less the mobile anchors we have placed.
In the origins of climbing did not exist, since normally the rope did not go through any insurance. They took advantage of the blocks and natural elements to halt a hypothetical fall.
When the first anchors appeared, pitons, these were equipped with a ring, but the climber had to unleash, to pass the string by the ring and to tie again.
Over time the carabiners appeared, which allowed the rope to pass without needing to unleash, but they were heavy, expensive and difficult to manufacture.
It was not until the manufacture of carabiners was standardized when they began to use timidly, as so far the climbers were used to climbing with a very, very small material and continued to use single carabiners as usual and also for An economic issue, since using express tapes meant doubling the number of carabiners and adding ribbons.
Little by little the use was imposed by a criterion of comfort, rapidity and because the prices were reduced gradually.
Its use allowed for faster climbing, which allowed to scale difficult passages without much effort - the rope was less scrubbing, which allowed better use of the length of the rope and allowed to climb longer runs. It was a discrete evolution, but that marked the ladder as we understand it now.
Nowadays there are countless kinds and models, but they all continue to maintain the basic shape of 2 carabiners united by a tape. What happens is that the carabiners have evolved and there is more variety: Big, small, with straight or curved trigger, also with wire trigger, anodized ... etc.
Now it is normal to wear express ribbons with a straight trigger carabiner to pass it through the climbing anchor and a curved trigger carabiner that makes it easier to pass the rope. Also used carabiners with wire trigger because they are lighter and have greater opening to pass the ropes comfortably.
The most successful carabiners incorporate tooth closure systems to prevent them from sticking in the anchors, especially when it comes to removing them.
As for the tape, 3 materials are mainly used: Nylon Polyamide and Dyneema.
Nylon and Polyamide are more durable and are usually used on simpler ribbons or those specially designed for sports climbing.
The Dyneema tapes are less durable, but lighter, thin and malleable so they are usually used for classical climbing ribbons, although it is not a fixed rule.
In any case, nowadays the straps are sewn to guarantee a greater resistance and do not use hoops so that the carabiners have limited their movement, except in the adjustable ribbons for mountaineering and classical climbing.
The most successful ones incorporate a piece of rubber or polymer to keep the carabiner fixed to hang the string, leaving the carabiner for the anchorage to move freely.
There are many lengths for us to use the one that best suits us, with the shortest ones prefering the sport climbing, medium-sized tapes for classical and sport climbing and long, extensible or adjustable straps for classic climbing and mountaineering . Or there is a fixed rule, but broadly it would be a classification.
In recent times, there have been some special models for the most disgruntled ... They consist of a rigid tape with a carabiner that remains open for itself and that closes when we place it in the anchorage. Simply complete the express tape with the carabiner you like the most. Its function is to get insurance that is a little further away with greater comfort and expose us less to a fall. This can give us a small dose of extra confidence and allow some climbing that is at the limit of our level.