Figure-of-nine loop (English Wikipedia)

Analysis of information sources in references of the Wikipedia article "Figure-of-nine loop" in English language version.

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archive.org

Marbach, Georges; Bernard Tourte (2002). Alpine Caving Techniques; A Complete Guide to Safe and Efficient Caving. English edition translated and adapted by Melanie Alspaugh. Allschwil, Switzerland: Speleo Projects, Caving Publications International. p. 72. ISBN 3-908495-10-5.