He Cokin hard-edge Y1 graduated yellow 2-stop filter can be used to enhance dull, flat, or hazy skies. The filter is densest at the edge and graduates to clear at the middle, with a hard-edged line of transition. Hard-edge filters are great for images with a defined horizon line. The filter is Rectangle so you can selectively choose where the yellow Edge sits in the framing of the image.
Cokin filters were invented by professional photographer Jean Coquin. By using square filters like Cokin's, which fit into a holder and are attached to a lens via an adapter ring, the photographer has a number of immediate advantages. There is no need to duplicate filters to fit different size lenses, up to three filters can be used at once to create unique effects, and the photographer has control over the positioning of the effect on the photograph.