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.
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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.
Step 4: Tightening, Retightening, and Locking
Slide the knot against the bundle. If it's not tight enough to keep from slipping, tighten it by holding it in one hand and tugging on the cords one at a time with the other hand. Notice that you can slide the knot out again, for instance to re-open the bag you just tied shut. For some applications, this may be as tight as you need it.
A springy bundle probably still feels loose. If so, compress the bundle and get a new grip on the cords. Rest your foot on the bundle, on the cords nearest the lark's head, and pull to slide the lark's head tighter.
To make the knot more permanent, you can lock it. Make an overhand in the cord ends. (Exactly like the first step in tying shoelaces.) With your foot still on the bundle, pull on the cords and move your hands apart until you're pulling in opposite directions. You have just locked the lark's head, and you can trust it to stay that way.(*)
But, let's say that later on you want to loosen the locked knot. Again put your foot on the bundle and cords nearest the lark's head, hold the two cord ends together, and pull. This makes a little slack between the overhand and the lark's head so you can untie the overhand.
In twine or small string, the locking overhand knot may be difficult to undo. Use a shoelace knot instead of an overhand, and it'll unlock easily.
(*) If you're using slippery cord, like polypropylene, an overhand may not lock securely. You may want to tie another overhand knot. For added security, wrap a short length of duct tape around the two cords, close to the knot.