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