The last episode of The Fresh Prince of Bel-Air is unusual in that it
features Arnold/Phillip Drummond from Diff’rent Strokes AND
George/Louise Jefferson (and Florence) from the Jeffersons, all
in-character, presumably with the shows’ producers’ permission.
This means that Diff’rent Strokes and the Jeffersons take place in the
same virtual world. By extension, the entire Jeffersons universe
(which includes All In The Family, Maude, Good Times, Gloria) is the
same as the Diff’rent Strokes universe (which includes The Facts of
Life, and Hello Larry).