Page 26 - Patrick Moreau-Marine "Knots How to Tie 40 Essential Knots"

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Round Turn and Two Half Hitches

The value of the round turn cannot be overstated.

While it is quite simple, it allows you to maintain the tension on a rope by
increasing arm force. And if the tension is too great, there is nothing wrong with
doing a second round turn.
This gesture has somewhat fallen out of favor with pleasure boaters, but it is
the first step of the most iconic sailing knot: a round turn and two half hitches.
An exceptional hitching knot, it is one of the few that can be tied and untied
under great tension.