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