It is preferable that the projecting parts {HEAD ~ 40b} are located on {TAIL ~ both sides} of one opening part 40a 
It is preferable that the projecting parts { HEAD ~ 40b } are located on { TAIL ~ both sides } of one opening part 40a
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH