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Monster Manual, p.305
Alignment: This line in the monster entry gives the alignment this creature is most likely to have. Every entry includes a qualifier that indicates how broadly that alignment applies to all monsters of that kind.
Always: The creature is born with the indicated alignment. The creature may have a hereditary predisposition to the alignment or come from a plane that predetermines it. It is possible for individuals to change alignment, but such individuals are either unique or rare exceptions.
Usually: The majority (more than 50%) of these creatures have the given alignment. This may be due to strong cultural influences, or it may be a legacy of the creature's origin. For example, most elves inherited their chaotic good alignment from their creator, Corellon Larethian.
Often: The creature tends towards the given alignment, either by nature or nurture, but not strongly. A plurality (40%-50%) of individuals have the given alignment, but exceptions are common.