Yes, an overhand knot. Yes, the same one you use to tie two ropes together to rappel. (If you want to get technical it's a “flat overhand bend.” previously known as the Euro Death Knot, of EDK).
Or, to really keep it simple just carry your cordelette completely untied. Want a loop? Tie with a flat overhand bend. Do you have three pieces of gear that are somewhat far apart? Tie a small overhand loop in each and, a.k.a. bunny ears style, to get more reach between three placements.
Hey, if you're happy keeping your cordelette pretty much permanently tied into a loop with a double fisherman’s knot, feel free to keep doing it that way, nothing really wrong with it. But, if you see somebody tying a cordelette as shown below, don't freak out about it, it's fine.
Just like if you were using it to connect two rappel ropes, make sure you've got a nice long tail at least 6-8 inches.
Note the striking resemblance to the Flying Spaghetti Monster . . .
Knot close up: Yep, that's your garden-variety overhand bend.
Hey, don't take my word for it. Here's a photo of an anchor made by AMGA Guide Dale Remsberg, taken March 2019. Notice the flat overhand bend connecting the cordelette ends.
and, in an email to me from internationally certified guide Rob Coppolillo, and co-author of “The Mountain Guide Manual”:
“I have my cordelettes tied with flat overhands right now....and I'm liking it. Easier to untie, etc. Only time I do not leave it tied as such, is what I'm using the cord as one big loop (as in, not tying it off as a distributed, redundant anchor material).
Indeed, the flat overhand starts rolling at relatively low loads, but in the testing I've seen it rolls once or twice, then quits....unless of course the load stays on it indefinitely.
Does this make sense? So, I guess I'd say, go for it with the flat overhand...but if you're using the cord as one big loop, maybe take the time to tie a double-fisherman's, if you foresee high loads.“