1, 6, 9 are figures which are half shaded by slanting lines. 2, 4, 7 are all divided into equal parts (either three or four parts) by straight lines and also have a black circle at the centre. 3, 5, 8 have similar designs and have their four corners shaded black.
2, 4, 9 have two similar elements intersecting each other. 6, 7, 8 have two different elements one placed inside the other. 1, 3, 5 have two different elements intersecting each other.