North Korea has issued a postage stamp to commemorate the United Nations International Year of Crystallography. The design features Wilhelm Roentgen, discoverer of X-rays, marking the 100th anniversary of the elucidation of the first crystal structures by X-ray diffraction; and a snowflake, surrounded by molecules of water, H₂O, marking the 400th anniversary of Kepler's recognition of the sixfold structural symmetry of water ice crystals. The stamp is issued with an explanatory companion as part of a special sheet that also marks the International Year of Family Farming declared by the Food and Agriculture Organisation of the United Nations.