The upper inside surface and the lower inside surface of {TAIL ~ the housing} define {HEAD ~ a group} of slots 
The upper inside surface and the lower inside surface of { TAIL ~ the housing } define { HEAD ~ a group } of slots
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH