RD stands for Reduced Depth - so this speaker fits into tighter spaces than the standard CCM663. A shallow cone and a redesign of the easy-to-mount QuickDogs frame reduce the depth to a more usable 90mm, while retaining everything that makes the CCM663 a great ceiling speaker. The bass / midrange driver's familiar yet discrete dark blue aramid fiber cone delivers an amazingly clean, detailed response. Combined with the pivoting aluminum dome tweeter for adjustable off-axis treble response and three-position EQ, this is one of the most versatile custom speakers available.