... ALL OF THEM.

As a musician (I've been a musician since I was three years old, and I'm now 33, so... yeah), I can personally attest and confirm that the musician mindset -must- be extremely disciplined and dedicated to a rigorous routine of rote rehearsal, which is an inherently lawful mindset. There is nothing chaotic about "one-two-three-four" over and over for hours on end, repeating one song to perfection.

As far as the other Bardic concepts, such as being a "free-spirit"... I would argue that there exists -no- class in D&D that is more free-spirited than Ranger... but -they- can be lawful.

Another one is the "thumbing their nose at society" arguement... well, that's pretty much what Rogues do... and -they- can be lawful.

Or this one: "It's in the nature of the magics they use" ... there is no difference between the chaos factor of Bardic magic vs Sorcerer's magics... yet -they- can be lawful.

There is no reason at all, whatsoever, that Bards cannot be lawful. It's a ridiculous restriction that makes -no- sense, and was put in place by someone in the initial design of NWN or at WoTC who obviously does not understand how music and the arts work.

I did some digging and it seems to be a case of "He said" aka:
Somebody once had an opinion
That opinion was referred to
That reference became a source
Somebody uses that source to verify his opinion
That source then became a fact.

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