In knot speak; we tie knots in a line, hitch a line to another item, or bend 2 lines together.  Even though,  a Square Knot could be viewed as a 'Bend', it is properly a knot, as so named.  They are pictured here as bends (2 different lines); but should be for both ends of a line to connecting to itself as a Knot; not to another line, to make a Bend.  The physical forces are different in 'Bending' 2 lines together, versus bending a line back to itself and carrying the pressure through a loop.  Rope work is fairly raw mechanics. 

Everything has an Equal and Opposite Reaction.  Rope, Knotting, Rigging (Hinge Forces) etc. study how to set that Equal and Opposite reaction to use against itself best to trap, or control the force.  So, the previous force is the active one, the rope just a passive responder.  In a choke of noose or half hitch, the load powers it's own choke onto itself etc.

A Square or 'Reef Knot' is for tying the same line to itself; as to seal a bag of all earthly belongings for months aboard ship, with other men.  In times way before the taming of electricity, much less civility sometimes!  To get a real feel, body sense of how some knots hold (Square, Surgeon's, SheetBend) some fail (Thief, Granny, Thieving Granny) try tieing a few feet of line around thigh joining ends loose and sloppy.  See how pulling outward on loop allows the knots to hold by seating, or walk out of line failing, as this test shows there mechanics of preference.  A tight Granny etc. can hold some, just not trustworthy for any time, thru jostling around etc.

Click Pictures for bigger picture with drawn explanations for more clarity of purpose.  Pop-up windows are adjustable size.

Warning:  Though the below knots show "Bending" different lines together; properly they are for making light duty loops in lines;   not for joining separate lines into a "Bend".   This is why the proper terminology is Square Knot, SheetBend etc.

	Everything has it's vocabulary to separate different items and instances.  Knotting is no different.  Clicking the picture at the right will give an enlarged view of the Basic Knot Terms.
	The Knots below, are   pictured as separate lines for clarity in their jointing only.  The Mechanics of pulls on the knots is different in a loop of line to itself binding a sack together, versus 2 separate lines joined under pulling load.  Click Picture for more information.
	The set of the simplest of knots to join 2 line ends; each with a 'bight' at the end to join to the other.  The line up shown, is incorrect, as each line would pull at the free end of the other line, and the the knot/bend comes apart.  Such a setup would form a "Thief Knot" like a silly 'landlubber' would tie, not a proper sailor's "Square Knot".
	This is the correct beginning of the simplest of knots, this one the Square Knot.  The line pulls track into each other, in the twist of a simplistic 'braid'.  Lines could easily slip apart, so the loose tails     must be secured.
	This is the Square Knot, not set, but true to form.  As you can see   not the most secure way of bending lines together.  Quite simply, a bight at the end of each line, set, together.  The Square knot, then     has only 2 parts, the 'braid' and the 'lock' on the tails.
	This is the Square Knot set.  Notice, that it does sit kind of 'Square'.  Also, tail of the Left line, is pinched against it's own standing part, as the Right line, does the same to itself.  This forms the 'lock'; to keep the 'braid' together, anything else won't work!
	The 'Thief Knot'; looks so similar; yet can fail instantly.  Notice how each line, does not really set against the other; but rather the Left line pulls against the loose working tail of the Right line; and the Right line, pulls against the loose tail of the Left.
	The 'Thief Knot' still sits 'Square'; but the Square Knot is rather    flimsy at best; and this slight imperfection of loaded line pulling against opposite loose tail, simply dooms the Thief Knot to failure.
	This is the 'Thief Knot' failing, as the loose tail of each line can't  take the full pull of the opposite line set against it, so is dragged out of the lacing.
	The 'Granny Knot' too looks similar to a Square Knot; the equal and opposite line pulls are properly matched against each opposite line's pull, not the free ends as in the Thief Knot.  But, here, the tails don't lock against their own standing parts, so can pull free.
	Here, the 'Granny Knot'; closer to being set, you can see, the tails won't set up against their own standing parts, the knot will not sit 'square', and will easily pull apart.  A Square Knot, is so   mechanically simple, on the point of failure itself, it must be made right.
	Here is the 'Granny Knot'; fixing to fail/ the lock slipping apart; as it doesn't sit 'Square', so that the free ends are pinched and locked against their standing parts.
	It is possible to make a 'Thieving Granny'; that pulls apart like a Thief Knot, and unlocks like a Granny Knot, getting both parts of the Square Knot exactly Wrong!
	The 'Granny' & 'Thief' knots show knot failures of the 'Square Knot'.  This 'Surgeon's Knot' is better than all 3.  An extra twist (to make a 'Round Turn' in 1 line around the other) is added to the main line pulls to upgrade the Square Knot to a Surgeon.
	The completed Surgeon Knot, still sits square, pinching the free ends against their own lines, and the line pulls are against each other properly.  The only addition/change is the Round Turn added, so the lines can grip each other more firmly.
	One of the old drawings with all 4 Knots; then going into SheetBends; where actually separate lines are joined together.  Click picture thumbnail to see larger view.  Click the pictures above for separate notes explaining them.