CREATE TABLE table_34811 ("Year of Issue" FLOAT, "Description" TEXT, "Weight" TEXT, "Diameter" TEXT, "Thickness" TEXT, "Edge" TEXT)

Which Thickness has a Diameter of 21.9mm?
SELECT "Thickness" FROM table_34811 WHERE "Diameter" = '21.9mm'
SELECT "Thickness" FROM "table_34811" WHERE "Diameter" = '21.9mm'