The Associated Press, one of the nation’s largest news organizations, said that it will, for the first time, attempt to define clear standards as to how much of its articles and broadcasts bloggers and Web sites can excerpt without infringing on The A.P.’s copyright.

The A.P.’s effort to impose some guidelines on the free-wheeling blogosphere, where extensive quoting and even copying of entire news articles is common, may offer a prominent definition of the important but vague doctrine of “fair use,” which holds that copyright owners cannot ban others from using small bits of their works under some circumstances.

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