  I revisit the phase structure of hot and dense matter out of quarks and
gluons with some historical consideration on the color deconfinement and chiral
phase transitions. My goal is to make clear which part of the QCD phase diagram
is under theoretical control and which part is not. I demonstrate that an
uncommon but logically possible scenario other than the standard phase diagram
cannot be ruled out. My emphasis is on the concern that one should correctly
understand what kind of phenomenon occurs associated with the phase boundary
line on the diagram. It is not quite obvious, in particular, where chiral
symmetry restoration plays a phenomenological role in the temperature and
baryon density plane except at the QCD (chiral) critical point.
