Coupling Ring
The coupling ring let's you attach a second filter holder allowing you to use more than three filters at once. You can also orientate square filters in different directions. Or move a Center Spot filter father away from the lens for a larger circle. Or other filters that are best used when placed far forward like Pastels , Diffusers and the Pre-shaped Frames to name a few
NOTE: Not for use with wide angle lenses.
What is vignetting ? How to avoid it when using filters ?
Vignetting can be caused by any extension in the front of the lens (filter, lens shade, etc...) and may result in producing black edges on the picture. Consequently, you must check that any accessory put onto the lens is large enough so that it does not appear in the angle of vision. For example, when using the COKIN CREATIVE FILTER SYSTEM, if the filter size of your wide-angle lens is Ø62 mm, you should go for the «P» Series, much larger, despite the fact that the «A» holder could physically fit this Ø62mm lens.
The Coupling Ring will also attach to the Modular Hood
Modifying a Cokin Holder
Unmodified Cokin P holders have a tendency to vignette on wide angle lenses (i.e., their front edges appear in the angle of view). That's because they have three slots, and that front slot is a long way from the front element of the lens. To "fix" this problem, buy an extra holder and modify it:
Make a cut through the middle of the second slot on each side of the holder. Discard the portions you cut off. Sand the remaining edge that sticks up on each side so that it is "flat" with the front of the first slot's holder. (See photo, below) 
