In another case, which is pending, The Associated Press sued the online news distributor All Headline News last year, saying that it had improperly copied A.P. articles.

The legal disputes are emblematic of a larger question that has emerged from the Internet’s link economy. The editors of many Web sites, including ones operated by the Times Company, post excerpts from competitors’ content from time to time. At what point does excerpting from an article become illegal copying?

Courts have not provided much of an answer. In the United States, the copyright law provides a four-point definition of fair use, which takes into consideration the purpose (commercial vs. educational) and the substantiality of the excerpt.

But editors in search of a legal word limit are sorely disappointed. Even before the Internet, lawyers lamented that the fair use factors “didn’t map well onto real life,” said Mr. Ardia, whose Citizen Media Law Project is part of the Berkman Center at Harvard Law School. “New modes of creation, reuse, mixing and mash-ups made possible by digital technologies and the Internet have made it even more clear that Congress’s attempt to define fair use is woefully inadequate.”

For now, Web sites are defining it themselves. Sites like Alley Insider and The Huffington Post are ad-supported businesses that filter the Web for readers, highlighting what they deem to be the most meaningful parts of newspaper articles and TV segments.

Alley Insider, according to its editor in chief, Henry Blodget, operates under a digital golden rule: “To excerpt others the way we want to be excerpted ourselves.” The post about Ms. Noonan’s column, including five full paragraphs, had explicit credits to the author and the newspaper, three links to the source and a direct encouragement to users to read the original column.

Alley Insider doubtlessly exposed new readers to Ms. Noonan’s column, and an unknown number of users followed the links to The Journal’s Web site. But others probably did not follow the link, meaning that Alley Insider alone  and not The Journal  reaped the advertising pennies from the excerpt.