"Polish knot, or double lark's foot: This knot is rarely used but can be quite useful as a mid-rope knot, placed directly in a carabiner. Compared to a girth hitch, it has an extra bight around the carabiner, preventing any slippage under the weight of the user. This isn't the case with a single girth hitch, which should not be used. The Polish knot is particularly convenient at rebelays because it is easy to adjust the slack in the rope."
I have used this knot several times and found it worked well. It's simple, easy and quick to tie. Certainly not appropriate for some, even many, situations, but a nice one to know.
Has anyone else used this knot?