Miller's knot

A miller's knot (also sack knot or bag knot) is a binding knot used to secure the opening of a sack or bag. Historically, large sacks often contained grains; thus the association of these knots with the miller's trade. Several knots are known interchangeably by these three names.

Constrictor makes a fiercer binding knot, but Miller's/Bag is suitable for most applications, and is easier to tie/untie. Miller's/Bag makes a great hitch, like the similar Ground-Line. Binding usage has force emanating from inside rope ring evenly, hitch usage has force input from one side, then reducing around.

Variations
As noted above, several other distinct knots have historically been known as miller's, sack, or bag knots; namely ABOK-1241, ABOK-1242, ABOK-1243 (Ground line hitch), ABOK-1674, ABOK-11 and their slipped versions. These fit the short description "two crossing turns – ends tucked under".

The following 3 knots do not fit the simple "two crossing turns – ends tucked under" definition but are very secure:
 * A slipped constrictor knot where the ends are passed through the opposing slips for security is quite secure as a bag knot. Unslipped, it is even more secure, but it may have to be cut at the bridge to open the bag.
 * A slipped strangle knot where the ends are passed through the opposing slips for security is also quite secure as a bag knot.
 * A bottle sling around a swirled and folded neck of the bag (elbow of the neck) is also a very solid and reliable bag knot. The fold provides the slight bulge a bottle sling needs for a secure hold. The bag may be hanged fully loaded by the ends of the Bottle sling tied together.

Tying
The common aspects of the most common bag knots are two crossing turns, and both ends tucked under some turns near the crossing point. Two ends, and two turns one can tuck under, gives a limited number of alternatives. All of these knots can also be made in a slipped form by starting with a bight and/or by completing the final tuck with a bight instead of the end.

To avoid ambiguity, versions of these knots that are not slipped are pictured below with the reference numbers found in The Ashley Book of Knots.