I thought I knew all the basic knots, but then I was taught this incredibly useful, simple knot. It's good for tying up the neck of a bag that you want to repeatedly open and securely close. It's good for tying around a springy bundle that will need tightening and retightening. And, it's easy to undo.
I've looked at knot literature but haven't found a name for this, so I named it. The basic knot is well known; it's called a "lark's head". Since all I've done is to slip the two ends through the lark's head to make a noose, I call it that.
WARNING: Don't use this knot when human safety would depend on it. Climbers and mountaineers have well-tested, reliable knots for such situations.
Step 1: Double the cord over
The lark's head noose requires enough cord to go twice around your bundle, plus a foot or two. Find the center and double it over. Sailors call this a bight.