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xkcd #2880: Sheet Bend
(imgs.xkcd.com)
Alt text:
A left-handed sheet bend creates a much weaker connection, especially under moderate loads.
WTF? the sheet bend is close to a square knot which is much more secure. Or a thieves' knot if you're looking for something that slips apart after limited tension.
A sheet bend is more secure for cords of two different types/diameters. A square knot is for cords of the same diameter.