Tree climbing is a sport that goes well beyond hanging out in the tree house in your grandma's backyard. Using gear and techniques that have been derived from rock climbers, cavers and loggers, tree climbing has become an adventure sport in its own right. There are three main knots used in tree climbing, but to know how to tie a knot you must first know the basics: the tag end of a rope is the end of the line, while the standing end is the rest of the coil.
A figure eight is the most common knot for tying a harness on to a rope. To tie a figure eight, hold a rope near the end and fold it so that you are holding a loop. Twist the loop clockwise twice so that the tag end and standing end are twisted together. Take the tag end and put it through the loop you've created, running from back to front. You should have something that looks like the number eight. Thread the tag end through your harness, and then use it to trace the knot you've already made, creating the follow-through.
A clove hitch is a knot that cinches down on itself and is used to raise or lower equipment to or from a tree. To tie a clove hitch around something, wrap the tag end over the object and bring it under the standing end to form the bottom loop. Then bring the tag end over the standing end and upward to form the top loop. Wrap it over the object again, but this time bring it under the line that connects the two loops, then pull tight.
A bowline is the fastest and easiest way to secure a rope to a stationary object. To tie a bowline, hold the standing end in your left hand and the tag end in your right. Make a loop by placing the tag end over the standing end, and hold the intersection in your left hand. Many people use a bunny rabbit method from here: the bunny (or tag end of the rope) comes out of the hole (from back to front), around the tree (the standing end of the line, again from back to front), and then goes back in to the hole (this time from front to back).