When {HEAD ~ a position} in the compared sequences is occupied by the same nucleotide base or {TAIL ~ amino acid}, then the molecules are homologous, or identical, at that position 
When { HEAD ~ a position } in the compared sequences is occupied by the same nucleotide base or { TAIL ~ amino acid } , then the molecules are homologous , or identical , at that position
OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH REL REL REL OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH