A ridge functioning as {TAIL ~ a weir} may be formed on {HEAD ~ the back plate portion} of the groove portion 
A ridge functioning as { TAIL ~ a weir } may be formed on { HEAD ~ the back plate portion } of the groove portion
OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH