The flat overhand bend (known for a long time with the now out-of-favor, tongue-in-cheek name of the "EDK", or "European Death Knot"), is still a solid choice for tying together two rappel ropes.
This knot has several advantages: Quick and easy to tie, easy to check if you tied it correctly, super strong and does the job, and it tends to rotate and ride up over potential obstacles, like rock ledges, where many other knots might get hung up.
But, let’s face it. It does look a little sketchy. How do you think it got that great nickname? I don’t care that 789,000+ happy rappels have been done on this knot with no issues, it just doesn't look that stout. This can be especially true if you are tying it in Less Than Ideal (LTI) circumstances such as:
With two ropes of very different diameters, such as a 10+ mm lead rope and a 8 mm twin rope that you're using only to make double rope raps
With extra heavy loads, such as in a rescue situation or with a haulbag / heavy pack
Maybe when it's wet, dark, cold, or all of the above, and you just need a little extra mental boost to be SURE your rap knot is bomber.
A knot that takes just a few extra seconds to tie, that uses a foundation figure 8 knot that every climber already knows, unties easily after loading, and can be a lot more confidence inspiring is called the Flemish bend, or a rewoven figure 8 bend.
(Knot geek terminology detour: a "bend" is the term for a knot that attaches the ends of two ropes together.)
This knot is fast to tie, works bomber with ropes of two different diameters, is easy to check if you’ve done it correctly because all climbers are familiar with a rewoven figure 8, and can inspire confidence when doing a sketchy rappel. And who doesn’t want a little more that?
Considering using the Flemish bend if you have a very clean pull for your rappel, without many obstacles. If your rappel is blocky and has potential places where the rope could get hung up, it's probably better to go with a flat overhand bend to minimize the chances of the rope getting caught.
Important note: The Flemish bend is completely different then the flat figure 8 knot, which looks similar but has both ends of the tail coming out the SAME SIDE of the knot.
The flat figure 8 knot is a death knot! The knot can roll in this configuration, has caused fatal accidents and should never be used for any reason when rock climbing.
So, let's learn how to tie it correctly.
Here's a nice video showing how:
and step by step instructions:
1 - Tie a figure 8 knot near the end of one of your ropes, with about a 1 foot of tail - no longer.
2 - With the end of your other rope, start retracing the figure 8 knot.
3 - Continue retracing the figure 8 knot . . .
4 - Complete it by giving yourself again about a 1 foot of tail. Important: the ends of the ropes should be pointing in OPPOSITE directions when you're done if you've tied it correctly.
5 -Dress the knot properly by pulling the slack out of all four strands - just like you would for a rewoven figure 8 tie-in knot.