There are numerous rules in the AD&D volumes about alignment. The most blatant of these are those restricting the varying classes. I've never really been a fan of restricting a class based off alignment, but then again I've rarely seen anyone actually play their alignment correctly unless they were doing so overtly (completely out of character and stating they were doing something strictly based on alignment). There are dozens of interpretations of alignment (almost as many as there are house rules for AD&D initiative) but only a few of these seem "right."
One of the biggest restrictions on alignment targets the paladin class. They are allowed to ONLY be Lawful Good. In this case I believe that the restriction is appropriate - so long as the gods of the world MATCH this alignment. I remember not having a paladin appropriate deity in the beginning, and everyone ended up worshipping Athena from the Greek pantheon or Tyr from the Norse Aesir (some of the few LG deities in Deities & Demi-Gods). Since the paladin's power comes from divine sources, having a campaign-specific deity for their alignment was sort of a necessity. Although we nominally ran adventures in Greyhawk, few if any of us actually knew of the gods presented therein except for the list presented in the back of the Glossography.
Assassins were required to be evil only, but had the choice of being Lawful, Neutral, or Chaotic. I viewed assassins as the antithesis of paladins, if not their opposite number. Where the paladin had to be truthful and forthright, the assassin struck from the shadows and was purposefully duplicitous and stealthy in his behavior by necessity.
Druids were required to be Neutral (and only Neutral). This seems to be a lame requirement. I'm much more in line with the 3rd edition method of keeping the druids at least partially neutral (NG, LN, CN, NE) to better reflect variations within the sect.
The AD&D rules also state that clerics cannot be True Neutral! Yet, there are a number of Neutral deities that have nothing to do with animals, plants, or nature. Do these have clerics? This was never stated anywhere. The most common examples of this were clerics of Boccob and Istus.
Finally, the ranger, who is supposed to be some form of Good alignment, may be the real antithesis to the assassin, although I don't see the contrast. Rangers are spell casters at higher levels who, despite being able to cast druid spells, are good-aligned. I'm not really sure if all these facts were taken from Tolkien or not, but the class has always bugged me, even more now that I know that the XP charts are completely wrong according to the original development of the class (4/3 xp gain from levels 1-9, normal thereafter).
Fighters and magic-users (including illusionists) have no restrictions on their alignment at all. Thieves must be non-good in alignment (but the occasional NG thief always seems to sneak in). Monks are supposed to be Lawful only, but how often will one be able to actually roll up a monk legally? The bard is an appendix class and not really run by many people so I won't even bother to mention it here.
So it seems that Gygax gave most of the sub-classes an alignment restriction. I wonder why? The sub-classes mostly evolved in the early game where there were only 3 alignments - the common 9 alignment system of modern times only came into existence with 1st edition AD&D. Originally the only two alignments used by player characters were Lawful and Neutral, since Chaotic was the alignment of the enemy. Having alignment restrictions in a game where only 2 alignment choices were relevant seems a bit goofy. So some of the sub-classes got altered to have specific alignment requirements and higher stats to qualify. Of course, when you make it a challenge to roll a certain type of character, you know that's the one everyone wants to play. Given the number of ILLEGALLY rolled paladins, druids, monks, and rangers in numerous campaigns, it appears that alignment is the ONLY restriction that sticks.
It's interesting to note that most people play all their characters as Neutral Selfish - in other words they seek out the best circumstances to fit their own style of play and seek to further their own needs over others. Some people see this as being True Neutral in alignment, but I see this as being more Chaotic Neutral (since Chaotics are only concerned with personal gain and benefits). You can tell when a player is abusing the alignment system when they quote the "I'm Neutral, I don't care!" line. This is a breach of alignment since the True Neutral stance is perfect balance between all the alignments where everyone gets along and no one steps on anyone else's toes. It's not that the druid seeks to avenge any wrongdoing done to HIM, but rather seeks redress against a person for despoiling nature or disturbing the balance. Neutrality is actually one of the hardest alignments to play because it is so difficult to remain absolutely neutral while adventuring in a dungeon. Some people take the eco-terrorist angle with druids, and this is wrong too according to their description in the Player's Handbook. They do not seek immediate vengeance, but rather plan and plot a subtle revenge over time, usually connected to the crime committed. In essence, druids believe in Karma - what you do in life is brought back to you in the end.
