By Nilsjohan - Own work, public domain, https://commons.wikimedia.org/?curid=11805173n.the act of folding paper (and sometimes film) so that one side of the material is folded onto itself and then back along the same crease in the opposite direction so that the opposite sides touchLewis 1959, 320[Table 7] Number of Double Folds to FailureCRI 1982, 514Our acid free file folders last a minimum of 1000 double folds during the MIT folding endurance test, not 200 folds like others on the market.ISO 5626, 19933.1 ¶ double fold ¶ One complete oscillation of the test piece, during which it is folded first backwards and then forwards about the same line.Mead and Baird 2003, 74Brittleness is defined as a book’s inability to undergo two double-fold tests.Mead and Baird 2003, 74, fn. 11The double-fold test is a test for paper brittleness made by folding a corner of a page over on top of itself, and the crease of the fold is pressed between the finger and thumb. The corner is then folded back the other direction along the same crease and pressed again. This is one double-fold.Doyle 2010A brittle book from the general collections came in today whose binding is holding on only by its spine lining. The paper and sewing are brittle and several pages are loose. It failed even a single fold of a (very very tiny) double-fold test, so it is too brittle to repair.
The double fold is used in testing the endurance of materials.