The Borromean rings -- no two circles are directly linked, but the three are collectively interlinked.  Cutting one ring frees the other two.  In terms of knot theory, a "Brunnian link".
For a monochrome version of this graphic, see File:Borromean-rings-BW.svg .
For a version of the Borromean rings depicted in triangular form, see Image:Valknut-Symbol-borromean.svg .
For extended Borromean patterns, see Image:Borromean-cross.png / Image:Borromean-cross.svg and Image:Borromean-chainmail-tile.png .
For other (more complex) three-component Brunnian links which are not equivalent to the Borromean rings, see Image:Brunnian-3-not-Borromean.png and Image:Three-triang-18crossings-Brunnian.png .

SVG version of Image:Borromeanrings.png .
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