Three impossible ways to combine four bars.
Impossible Joinery
See also
Freemish Crate, Impossible Figure, Penrose Triangle, TriboxThis entry contributed by Margherita Barile
Explore with Wolfram|Alpha
References
O'Beirne, T. H. Cover of Puzzles and Paradoxes. London: Oxford University Press, 1965.Referenced on Wolfram|Alpha
Impossible JoineryCite this as:
Barile, Margherita. "Impossible Joinery." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/ImpossibleJoinery.html