The Petzl String is a little rubber piece that attaches to the end of a quickdraw to keep the bottom biner correctly oriented to clip the climbing rope into. It also offers some protection from the end of the sling wearing out. While most quickdraws come with a rubber attachment, the String is by far the most durable and does a great job of holding the bottom carabiner in a rigid position for easy clipping. They only break after tons and tons of heavy use. They are ideal for replacing the worn out Strings on the Petzl Express Quickdraw but we also recommend using them on just about any quickdraw. For example, we love the Wild Country Helium Quickdraw but found that the rubber piece broke of quickly. So we replaced that rubber piece with the String. While the string overhangs a little because the Helium webbing was narrower, it is still a nice replacement. Not only is the string our favorite, it is really one of the only options if your quick draw attachment point breaks. If you are considering upgrading your quickdraws with this, you might also check out buying the complete Petzl Express Sling which is a thick runner that comes with the string. It is our favorite quickdraw sling because it is comfortable to grab and very durable.
WARNING: Do not use String on an open sling - only use on a quickdraw sling or "dogbone". Incorrect use has resulted in death.
It comes in two sizes: L and XL. We recommend the L for most applications.