NaCl planes.
You can easy break NaCl crystall along (100), (010) and (001) equivalent planes (blue color). But you cannot break it along (110) plane (red color) because you have to destroy more atomic bonds (black lines). (111) atomic plane (white color) contains charged atoms of one kind (Na+ or Cl -) and you cannot break crystal along this direction due to electrostatic attraction.
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Crystal models     updated 18 Oct 2011