The Figure 9 knot
The figure 9 is pretty much the same as figure 8 on a bight. The one difference is you make an extra twist before you tie it.
Why might you want to use the figure 9?
Because the figure 9 uses extra cord, this raises your master point a bit higher, which may be more ergonomic. Check the photos below, you can see how the figure 9 gives the highest master point.
Admittedly this is fairly rare, so most of the time the figure 8 is going to be the better choice.
A possible downside: the figure 9 may not be as recognizable as the figure 8, so you might have to explain to your partner what the hell you're doing. =^)
The sling I'm using is the Edelrid Aramid cord, my favorite for anchor building. Strong, very durable sheath, and easy to untie after being loaded, no matter what knot you use. Here's a longer article about these slings.
Lots of people think that figure 9 is easier to untie then the figure 8, but depending on your loads and the rope/cord you're using, that may not be true. Check out this video below from The Rope Access Channel on YouTube. He puts about a 480 kg load on both knots and tests which is easier to untie. Hint, it's the eight.
In the video below, the test goes from about 2:45 to 6:00.
