There isn't any MD-84.
The sub-variants in the MD-80 Family are -81, -82, -83, -87 and
-88. The MD-80 Family is superseded by the MD-90 Family. The MD-95
is now known as Boeing 717.
The MD-80s, MD-90s and B-717 are basically a modernized stretch of
the DC-9s.
