The filter elements are surrounded by a frame which has two different types of projections at each of the shorter sides. This allows the frame and thus the filter be installed in an only direction possible. There is no difference between left and right. When the filter in the box so committed, as the noses it provide the filter element as printed on the frame is exactly the wrong way round. The Air Flow arrow then from top to bottom, so air from top to bottom. At E39, the air flows but from the bottom up! The filter box is sealed off upwards with lid and seal the air which is located vertically above the built-in filter is thus fed into the interior. Air beneath the filter is vertical the dirty outside air. When installed as required by the nose, now the Air Flow arrows are exactly opposite to the actual direction of flow.