LEE Filters Holder System - Foundation Kit
The Foundation Kit is at the centre of the LEE Filter holder system and is primarily designed to take standard 100mm filters. The holder is supplied in component form to allow you to construct a filter holder to your individual needs. The holder clips easily onto the Adaptor Ring and can then be rotated to position filters to your exact requirement enabling optimum use of graduated and special effects filters. It allows 1,2,3 or even 4 filters to be attached to almost every lens on every format camera.
Source : http://www.leefilters.com