Back in the day, camera manufacturers standardized on a single (threaded) filter size for their lenses but no mare. If you add lenses from independent lens manufactures to your system, you can easily find yourself owning lenses requiring many different filter sizes.
Modular filters overcome this obstacle by letting you to purchase one filter that fits all of your different lenses. That means you only need to own one ND, gradient, or polarizer filter and it will fit all your lenses. These filters slip into a holder that uses adapter rings allowing them to fit different lenses and even different format cameras. All you need is a single, inexpensive adapter ring for each of your different lenses thread sizes. The sizes of filters and the type of adapters used vary by modular filter manufacturers,but some companies filters are compatible with other systems.
Modular filters are square or rectangular, although companies, such as Cokin, Hi-Tech and others offer round filters as well. These filters are also more than likely to be made out of plastic. These plastic filters are typically constructed from the same kind of optical resin material used in eyeglasses.
Cokin’s original A filter system uses 75mm x 75mm filters. The larger P (84 x 84mm) filter system was designed for medium format cameras, but an A/P adapter lets you use the A filters in a P holder so it’s not necessary to buy P filters if you already own equivalent A versions. Tip: When using A-sized filters on shorter focal length lenses some vignetting may occur so watch out for this problem on your LCD.
Cokin offers the large (6.7 x 5.1) inches X-Pro size that uses a holder similar to the A and P series but with more rugged construction. Cokin then launched their grande 130mm wide Z-Pro series. A Z/P adapter lets you use Z filters on a P Holder and the Z/P Wide Angle Adapter lets you use Z filters on a P Wide-Angle Holder.