Say you have a standard cordelette tied in a big loop, and you want to use it to build an anchor on a stout tree. If you pass the cordelette around the tree and tie it off in an overhand knot, it might seem that you have a shelf on the top two strands, just like you have when you clip two separate pieces of gear.
But be careful, it’s not what it appears. If you clip the two strands like you would for a gear anchor, there is a 50% chance you're going to be on just one strand of the cord, and therefore not redundant.
See the photos below.
I have to say, I was scratching my head a bit when I first heard of this. I had to set it up on a tree in my yard to see for myself.
After you read this, you may think that any anchor failure like this in the real world is so extremely unlikely that you're not going to worry about it. Well, that may well be true. However, this is not a very intuitive thing to grasp for most people, and I want to illustrate best practice in all aspects of anchor building. So, absorb it if you want and toss it if you don't. :-)
Long cordelette looped around a tree, tied off with an overhand knot and locking carabiner as the master point. Perfect.
Hmm, thinks the climber, I think I want to add a shelf. How about I clip another locker to the two strands coming out of the top of the master point? Well, doing this is not a lethal mistake, but if you think you’re getting redundancy on two rope strands, you only have a 50% chance of doing so.
As it's shown here, the top carabiner appears to be clipping two different strands, but it's actually clipping the same strand twice - not redundant, whoops! You can't see it in the photo without a video walk-around the tree, but you can easily set it up yourself to see what I mean.
Here, the black carabiner is clipped correctly to both strands coming out of one side of the masterpoint knot. It's clipped to both strands, giving redundancy. Yes, if you're a long time cordelette user and have always used it to clip single pieces of gear, this photo below is probably giving you a minor freakout. Trust me, it's right, and set it up for yourself to see how it works. You can do it inside around a chair or pretty much anything.
(Of course, there's no reason to really use the shelf at all. In most cases, it's an optional nice thing to have but certainly not a mandatory part of the anchor. So, if this post is making your eyes cross, you could go back to the photo at the top, with no mention of the shelf at all, build an anchor like that and be just fine.)