The Cokin Full transition 3-stop graduated Neutral Density filter is like having two filters in one. The filter starts with a 3-stop ND on one edge and transitions across the whole filter to 1-stop ND at the opposite edge. This creates a base of 1-stop of total light reduction across the whole image then provides an additional 2-stop graduated ND for the top half of the filter. This is great for scenes that require a total light reduction but also requires brighter areas, like skies, to be reduced further.