Sheet Bend & Double Sheet Bend
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The Sheet Bend is a general utility bend valued for its simplicity and ease of untying yet avoided for important or severe work due to its lack of security.  It is the same general knot form of the Bowline, and can be tied in the same quick one-hand twist method that helps to prevent tying the knot incorrectly.  When line sizes aren't identical, the larger line should assume the "U"-shaped part. The Sheet Bend is also typically one of the primary knots used in net making.

In small stuff it may be convenient to tie a Sheet Bend by tying a slipped overhand knot (slip knot) in one line, and then place the other line though the noose.  When the standing part and free end of the slip knot are pulled, either a Sheet Bend will emerge or a relative of the Sheet Bend.
The Double Sheet Bend is an effort to make the Sheet Bend more secure.  However, if security is important, it's probably prudent to use a Zeppelin Bend (Rosendahl Bend) instead. 

Similar to the Sheet Bend and Bowline analogue, the Double Sheet Bend can be seen in the loop form in the
Double Bowline.

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It should be noted that a bend and its loop form do not necessarily share the same security characteristics.