The groove includes a central concave portion and {HEAD ~ two support surfaces} extending downwardly from {TAIL ~ respective sides} of the concave portion 
extending downwardly from
The groove includes a central concave portion and { HEAD ~ two support surfaces } extending downwardly from { TAIL ~ respective sides } of the concave portion
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH