Figure-eight loop

Figure-eight loop (also figure-eight on a bight, figure-eight follow-through, Flemish loop, or Flemish eight) is a type of knot created by a loop on the bight. It is used in climbing and caving where rope strains are light to moderate and for decorative purposes.
The double figure eight is used to put a loop in the end of a rope, or around an object. It is relatively easy to tie and is secure, but can become difficult to untie after heavy loading, and can jam badly in any rope type.
Tying methods.
On a bight.
A figure-eight loop is created by doubling the rope into a bight, then tying the standard figure-eight knot.
In climbing, this knot is used to save time when repeatedly attaching the rope to climbing harnesses, using locking carabiners, such as when a group of people are climbing on the same top-rope.
Follow-through.
Alternatively, to tie the knot directly around an object, the follow-through method must be used. 
Climbing.
This is the standard method for attaching a rope to a climbing harness. 
Often an additional Strangle knot (what is half of a grapevine knot) "backup knot" is tied in the tail the figure 8. This is not required for the knot's integrity during climbing, but could prevent ring-loading failure if belaying from the rope loop (instead of a dedicated belay loop). It also ensures that adequate tail length has been included, and gets excess tail out of the way. If the finish knot is not included, the tail should be 4 to 8 inches long. The tail can also be tucked back into the knot, called a "Yosemite finish" or "Yosemite tuck". This holds the bottom loop open, making the knot easier to untie after falling, but also making it weaker in a ring-loading configuration. 
The diameter of the loop should be kept small, to avoid being caught on protrusions while falling, or clipped into accidentally while lead climbing. A well-dressed knot has a symmetrical appearance, with the strands parallel through each curve.