The Cokin X-pro series filter holder allows photographers to attach up to 2 square or rectangular filters. For example, it is common for photographers to use a straight neutral density filter like a 3-stop to control the motion blur of a waterfall then combine it with a 2-stop Graduated Neutral Density to control the detail in the sky. The end result is a beautifully captured single exposure.
Cokin also manufactures a round Circular Polarizer (X164) that is specifically designed for the X-pro series holder. Cokin created a special slot on the holder closest to the lens that accepts the outside edge of the round filter. The slot allows you to insert the filter and then rotate it to achieve the desired polarizing effect. With this filter attached you are able to add one more additional filter to the front slot of the holder.
The X-pro series holder will work with Full-Frame lens focal lengths 15mm and greater. If you use the holder on a lens that is wider than 15mm there is a very good chance that you will see the holder in the corners of your pictures, This is also called a vignette or vignetting. It is also possible that you will experience vignetting even at 15mm if the holder is rotated off center axis.
The X-pro series holder also requires an adapter ring, purchased separately, that will allow you to attach the holder to your lens. It will screw onto the front of your lens just like a regular camera filter does. The adapter rings are available in different sizes from 62mm to 112mm. To determine which adapter ring(s) you need simply remove your lens cap from your lens and turn it over. 98% of the time there will be a number on the backside of the cap. That is your filter size. If that does not work you can go to the original lens manufacture's website or instruction manual specifications section and you will find your lenses filter size there.
It is highly recommended that you remove any other filters from the lens before mounting the adapter ring and holder. Additional filters will increase the chance of vignetting especially on wide angle lenses.