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', for it ties 2 ends together.  A Square Knot is properly a knot, as so named; for it is meant to tie the ends of the same line together.  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. 

 

Square	Surgeon	Proper Knots
		Tails/Working Ends on the same side.  A Surgeon is just an advanced/ 'Round Turn' Square Knot.  W.Ends, still finish on the same side.
Granny (Bad)	Thief (Bad)	Bad Knots
		Tails end on different sides of the lines.  These are both false Square Knots.  A Thief is extra sneaky, as it sits 'square' like a Square Knot!   A key factor here is that a Square Knot sits stabilized by holding itself square; these 2 destabilize and slip.
SheetBend	SheetBend (Wrong)	SheetBends
		This carries over into the SheetBend, the Working Ends, should finish on the same side of the lines

If you go to bind/wrap something with 1 wrapping turn around, and simply seat 1 , then 2 twists of the ends together, you will probably make a Granny Knot; but would want to make a Square Knot.  The tendency is for you to make both twists overhand or underhand; so that they match.  What ever you are most likely to make on the first twist of the ends, you even quicker move to duplicate for the 2nd twist of the ends together, resulting in a Granny Knot.  To make a proper Square Knot, the tail ends must seat together, because they are opposing overhand knots, not matching.  The knot should not be used as a bend, also; should be laid against a package etc. to stabilize it from spinning etc.; not used in mid air.

 

Square Knot	Granny Knot	Make a Square Knot, not a Granny Knot
		A Square and a Granny start out the same; 1 end taking a turn around the other as pictured.  The black arrow shows the next move.  Placing W.End against it's own S.Part (Red to Red), properly makes a Square.  W.End laid against the other end's S.Part (Red to White) is a goof, and makes a Granny!
		As shown, the Red W.End lays neatly against itself, and then White will do same (naturally) for Square Knot.  If Red W.End lays against White's S.Part, White must match too, resulting in a Granny Knot.
		In the Square Knot, the W.Ends sit against their own S.Parts, to seat under pinching lock of opposite line's bight.  The W.Ends are on the same side.  A Granny Knot, doesn't lock the W.Ends against their own S.Parts, so can slip.
Surgeon's Knot		Making a Surgeon's Knot
		A Surgeon's is much better, it just starts with an extra turn to twist lines together.  Such a Round Turn upgrades any simple knot immensely.  You must still get the W.Ends to finish correctly against their own S.Parts.  Otherwise, it could fail, if it finishes like a Granny Knot.
Square Knot	Thief Knot	Make a Square Knot, not a Thief
		A Square Knot and a Thief Knot start out the same.  If you take a W.End over the other S.Part, you are making a Square.  But, Take the W.End over the opposite W.End, and you are making a Thief Knot.
		After Red Working End crosses White's Standing Part (Square) or White's Working End (Thief), it continues around underneath and comes out the opposite side
		Then the Working Ends will next properly lay "Squarely" against their own Standing Parts.  Everything the same, except for the start, so tails finish different on the 2 knots.
		Square Knot and Thief Knot look very similar.  The Square finishes with both ends on the same side, a Thief Knot does not.
		A Square Knot matches the pulls against each other's S.Part.  A Thief Knot looks similar, but the S.Parts, pull the W.Ends of the Opposite line out.  Because the main pulls pull on the free ends.
		If you have a Bus pulling, and take enough wraps around a spar, a Baby could hold back the mighty Bus with the Blue line.  Most knots take what the Baby could hold, and pinch it behind the main bus pull to secure (Red Arrow).  (click Pic. to enlarge)
Square	Thief	2 parts to a Square Knot
		A Square Knot is just 2 bights slipped together.  No reduction of forces.  For a Square we have to settle for just matching main pulls /Bus vs. Bus.  A Thief, Puts the Bus pulls directly against a Baby hold (W.End/ Free Tail), and pulls the tail free!
Thieving Granny	Surgeon/ Granny
		It is possible to make a 'Thieving Granny'; and a Surgeon with a Granny finish.  The Granny finish can be put on any of the forms; causing failure.
		Warning:  Though the above 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 vs. SheetBend etc.  They are shown in different color lines for clarity only.
		If we keep 1 line just a bight, and make the other a half hitch to join the ends, we make a SheetBend.  We now don't have the lines just slipped together with 2 bights; but have an actual lock on the lines (but not good enough to trust your life to!)
		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.