Each of the sides has {HEAD ~ an empty space frame} by outer edges of {TAIL ~ the housing side} 
Each of the sides has { HEAD ~ an empty space frame } by outer edges of { TAIL ~ the housing side }
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL