RD stands for Reduced Depth – so this speaker will fit into tighter spaces than the standard CCM663. A shallower cone and a redesign of the easy-fit QuickDogs frame reduce depth to a more usable 90mm,while retaining everything that makes the CCM663 such a great in-ceiling speaker. The familiar-yet-discreet dark blue Aramid fiber cone of the bass / midrange drivers a stunningly clean, detailed response.This,combined with the pivoting aluminium dome tweeter for adjustable off-axis treble response and three-position EQ, make this one of the most versatile custom speakers around. If preferred, the supplied round grille can be replaced , Square CCM663 grille.