Colour graduated glass filters create an effect in the top part of the image while leaving the bottom undisturbed. Accordingly they are most commonly used with skylines. Dramatic effects can be achieved using coloured graduated filters.
Colour graduated glass filters are available in Soft edge, Hard edge or Razor edge varieties. Soft edge grads have a smooth transition from clear to the colour. Hard edge filters have a more defined hard lined transition and should be used for longer lenses. For extremely long focal lengths a Razor edge grad, providing a very highly defined transition, may be ordered. Interesting effects can be achieved using two different graduated filters, (one upside down) one of which affects the top half of the image with the other affecting the bottom half.
Please note that when ordering graduated glass filters in Panavision (4x5.65) size, there is a choice between horizontal (landscape) and vertical (portrait), depending on which way you will be using the filter in the matte box.
For more information, please see this Colour grads PDF
The following colour graduated glass filters will give a cooling