You are correct that knight and cavalier are the same archetype/theme.Having played this game since 1979, I'm well familiar with the inclusion of the Cavalier in 1985 (1E). Also, I have memories of a Knight being brought in somewhere in the mid-2000's (3.5E).
Aren't these classes just variations on the same theme, but with a different name & different "feel"?
I guess what I'm trying to say is, can anyone successfully justify the existence of BOTH a Cavalier AND a Knight class in the same game? If so, how?
So are the classic cleric and the paladin: in his PHB Gygax describes the cleric as inspired by the fighting orders of the crusades, who were - in self-conception - the pinnacle of knightly chivalry, which is exactly the same archetype as a paladin. Both classes are heavy armour-wearing, heavy weapon-wielding front-line combatants who can perform miracles, particularly healing and turning away evil spirits.
If there is nevertheless a justification for having both traditional clerics and paladins in the game, it will have to be at the level of mechanical differentiation rather than archetype. (Eg in AD&D a paladin has better attacks and hit points, a cleric has better miracles.)
The same sort of justification could be used for having both a knight and a cavalier in the game: same archetype, but mechanical differences that matter to D&D play. (Eg the knight could be a 'defender', the cavalier more of a damage dealer by means of mounted charging.)
In AD&D, 'chevalier' was level title for both paladins (in the PHB) and cavaliers (in UA). Cavaliers also had various 'knight' level titles (from around 4th level, I think) while the ranger became a 'ranger knight' at 9th level.there was never an edition (that I know of) which included both a Cavalier class and a Knight class.
In 4e, as [MENTION=996]Tony Vargas[/MENTION] pointed out, there is a 'knight' sub-class of the fighter and a 'cavalier' sub-class of the paladin. From the point of view of archetype they are very close - both use heavy armour and weapons and both are eminently capable of defending their allies. The paladin (cavalie) has some miracle-working (healing others) and can do radiant damage on attacks (which hurts undead), and the fighter (knight) has a bit more battlefield control, but they are very, very similar classes. (Much more similar than the fighter and paladin in the 4e PHB.)