The half hitch is a simple overhand knot

Overhand knot

The overhand knot is one of the most fundamental knots and forms the basis of many others including the simple noose, overhand loop, angler's loop, reef knot, fisherman's knot and water knot. The overhand knot is very secure, to the point of jamming badly. It should be used if the knot is...

, where the working end of a line is brought over and under the standing part. Insecure on its own, it is a valuable component of a wide variety of useful and reliable hitches, bends, and knot

Knot

A knot is a method of fastening or securing linear material such as rope by tying or interweaving. It may consist of a length of one or several segments of rope, string, webbing, twine, strap, or even chain interwoven such that the line can bind to itself or to some other object—the "load"...

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One instance where a half hitch stands on its own without additional embellishment is when added to a timber hitch

Timber hitch

The timber hitch is a knot used to attach a single length of rope to a piece of wood. This knot is easily undone after use.To make the knot, pass the rope completely around the wood. Pass the running end around the standing part, then through the loop that you have just formed. Make three turns...

 to help stabilize a load in the direction of pull. A timber hitch is tied on the far end of the load to bind it securely and a half hitch made at the forward end to serve as a guide for the rope.

The knot is attractive to the eye and so is used decoratively for French whipping which is also known as half hitch whipping.