The Most Important Knot You will Ever Learn.
It might surprise you to learn that as late as the writing of this post that their is still confusion about what knot to use when tying in to your climbing harness, whether in a gym or out on the crags. One of the latest trends in many climbing gyms is not to tie in at all, instead they have fixed knots where the climber simply uses a carabiner and clips into a loop of rope. This technique might work if the gym is using any of a number of approved auto belays with fixed carabiners, or it may be o.k. if you are climbing any of a number of mobile climbing walls, also equipped with auto belay devices. However, you should not rely on a fixed knot in a climbing gym for at least two reasons:
1. A fixed knot will weaken over time and will become less effective.
2. Not all gyms use fixed knots and this may limit your experience of going to other climbing gyms.