ND Grad filters are half grey in colour and half clear. They are mainly used in shooting landscapes with bright skies and they reduce the amount of light reaching half of the picture. As a cameras sensor rarely records the correct exposure of ground and sky together, they are used to improve the balance between the main subject and the foreground/background. The grey portion of the filter is neutral in colour and does not affect the overall colour balance of the scene being photographed. Cokin offers a choice in hardness of the graduation between the grey and clear section and the filters can be adjusted vertically and by rotation to suit each application. The 121L is a "Soft Grad" with the graduation occurring in the top third of the filter.
 
Filter holder and adapter ring are sold separately. 
 
All Cokin filters are manufactured from CR-39 organic glass which was originally launched for the vision lens industry. 

CR-39 organic glass boasts several major features:
 
Extra lightweight
Highly resistant to shocks, meaning that photographers, videographers and cameramen can handle the filters in complete confidence and safety
Excellent optical transmission and high compatibility to colouring, making Cokin filters the proven choice of photographers worldwide

The Cokin Creative Filter System consists of three main components: A filter holder / Adapter ring and a Filter.

 Screw the adapter ring onto your lens.

Slide the filter holder on the adapter ring until it snaps in place. The filter holder is now solidly attached to the lens, yet it can rotate both left and right.

Slide the filter into one of the filter holder slots. The one closest to the filter holder housing is the narrowest; it allows the use of round filters with notched edges (polarizer, star filters..). The two central slots are intended for square filters. The outer slot can be used either for an additional filter or for a coupling ring.