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.
Step 2: Form the lark's head
Coming from below the bight, hook your thumb and forefinger over the two sides. Reach outward and downward and pinch your fingertips together underneath the doubled cord.
You've made a lark's head. Move the crossing cord a little farther from your hand, so there's a clear opening through the two half-loops.
Step 3: Form the noose
Slip the two ends of the cord through the opening we were just talking about. Pull the lark's head tight so it grips the two cords. Find something that needs constricting, and put the noose around it.