3e SRD:Diplomacy Skill
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The character can change others' attitudes with a successful check. In negotiations, participants roll opposed Diplomacy checks to see who gains the advantage. Opposed checks also resolve cases when two advocates or diplomats plead opposite cases in a hearing before a third party.
Generally, retries do not work. Even if the initial check succeeds, the other character can only be persuaded so far, and a retry may do more harm than good. If the initial check fails, the other character has probably become more firmly committed to his position, and a retry is futile.
Charisma checks to influence NPCs are generally untrained Diplomacy checks. If the character has 5 or more ranks in Bluff or Sense Motive, the character gets a +2 synergy bonus on Diplomacy checks. These bonuses stack.