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The eight-link is a type of chain connector that used to be readily available at most hardware stores, but seems to have been replaced by maillon-type connectors. My father had one lying in an old tool box. The red one in the picture one is designed for half-inch chain, so it is a bit too large to make an ideal ascender, and having the wrong size probably distorts my test results. In 2007, William Greenwald sent me two 3/8 links that work better, these are the ones shown in the bottom photo.
Gerald Wood (inventor of the whaletail) invented the eight-link knot in 1967 after experimenting with the snap hook knot, and published his idea in the December 1967 NSS News.
Bob Thrun tested it when writing Prusiking. Bob's tests indicated that the eight-link knot did not move as freely as an rbs knot - which is one of the better ascender knots. Since eight-links are heavy and no longer common, I suspect that the eight-link knot will remain a historical curiosity.
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Bob Thrun (in Prusiking) reports that William Greenwald invented the Hedden Eight-Link Knot in 1968, after reading about the Eight-Link Knot. Basically, the Hedden Eight-Link Knot is a Hedden Knot with an eight-link inserted to fascilitate loosening the knot. To do so, pull the eight-link down, then push the entire knot upward. The method works well, but the knot is a bit too tall for my taste, and it has a bit of stretch to it. The knot also takes too long to tie - there are numerous simpler alternatives.
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