This is taken from my book, Weaving for Beginners, on page 352. I find it very useful.
Lark’s head knot Figure 582
The lark’s head is an enormously useful knot and quick to make once you’ve mastered the thumb and finger pinch action. It’s great to use when you have an intact circle with no ends available, such as a rubber band, or when you have a loop. It also can be made with a length of string folded in half (with two tails). It’s used to attach things—what is to be attached is encircled and held secure in the knot’s loop, which draws up and holds the object tightly. It is extremely secure and can’t come undone.
In fact, practically whenever a loop is involved, the lark’s head is useful. Rather than list all its uses, I’ll just urge you to learn to make it.
To make a lark’s head knot:
There are two ways to tie this knot, depending on the situation: when you have a loop, or when you have a length of string you can fold
in half to make a loop.
Method 1, when you have a length of string.
Fold the string in half and take the tails around a stick, rod, or a ring, and bring the tails through the loop.
Method 2, when you have a loop or a circle.
1. Hold tension on the tails of the loop or the side of the circle with the left hand.
2. Insert your right thumb and forefinger into the loop, spread them wide apart to spread open the loop.
3. Take your finger and thumb around the taut parts of the string until your finger
and thumb touch each other.
4. Keeping the finger and thumb together in a pinch, ease the taut strings into and
through the original loop as the loop folds over your pinch.
6. Put a stick in where your thumb and forefinger are. The loop can be made large to
hold a big stick if needed, such as when you put a warp onto the kitestick.
To release the knot:
Simply take out the object that the knot is looped around.