Google is inviting users to participate in its Knol online encyclopaedia service.

Knol allows ordinary users to contribute their own articles on various subjects, and experts to contribute full articles in their fields of speciality.

The service had been in closed beta since it was announced in December 2007.

The concept behind Knol has clear similarities with Wikipedia, but with one glaring exception.

Knol will place the author's name on each article, allowing readers to see who wrote the piece, and will allow multiple articles on the same topic.

The anonymity of Wikipedia has long been a point of contention with users. Political groups, PR firms and overzealous users have been involved in numerous instances of Wikipedia entries being inappropriately modified.

"The key principle behind Knol is authorship," wrote Google product manager Cedric Dupont and software engineer Michael McNally.

"Every knol will have an author, or group of authors, who put their name behind their content. It's their knol, their voice, their opinion."

Third parties will be allowed to edit and amend articles through a process called 'moderated collaboration' in which the author will be required to approve any edits to a page.

Google will also allow users to monetise their entries with an opt-in system for the company's AdWords programme. Authors who choose to allow the ads will be paid a share of revenues.