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