A keelstepped mast must be securely fixed in the deckring without any movement. This in order to prevent damages to the mast and to obtain the required integrity of the complete rig system. The mast must have an even curvature (convex forward) right down to the mast heel which is achieved by a correct mast heel location and orientation of the mast in the the deckring. 

This rubber wedge is designed for C- and F-sections and associated deck rings. For information on what mast wedges to use for your individual mast and how to fit them, go to the Seldén website and Yacht Catalogue, Version 8, page 46. 

Basically, put the aft wedges in place and then put the forward rubber wedge in the composite wedge and tighten it down. Wet the rubber with water/soap and it will slide down easier.