Page 247 - Buck Tilton - Outward Bound Ropes, Knots, and Hitches 2 ed.

Thread the working end of the second rope into

the figure 8 knot in the first rope, as shown in the
photograph.

The figure 8 bend is created by weaving together two figure

8 knots. Climbers tend to prefer this bend because of its
security as well as the strength of the interwoven working

ends. If heavily loaded, this knot may prove impossible to
untie if the ropes are of approximately the same diameter. It
does hold nearly as well in ropes of dissimilar diameter.

Despite its security, climbers often leave the working ends
long enough to back up the knot with stoppers, adding even

more security (not a bad idea when your life may depend on
your knot).

Clifford Ashley, arguably the greatest knot-man of all
times, called the figure 8 bend a Flemish bend and found it,

reportedly, a bothersome knot to deal with. This knot may
also be used by campers and boaters.