US judge rules most of Sir Arthur Conan Doyle's creation is in public domain but details from post-1923 stories remain off limits

Prospective authors of Sherlock Holmes fan fiction take heed: under a new court ruling, you may write that Sherlock Holmes was a cocaine-addicted martial arts aficionado cohabiting occasionally at 221B Baker Street, with a friend called Dr Watson.

You may not, however, freely describe Dr Watson's own athletic background, the juicy fact of his second marriage or the circumstances of Holmes's retirement.

A US district court in Illinois found itself wading into the details of the fictional detective's imaginary life this week in a copyright ruling on a forthcoming collection of original short stories featuring Holmes characters.

An editor of the new book, In the Company of Sherlock Holmes, asked the court in effect to enlarge the elements of the Holmes story that are in the public domain. The court reinforced the public domain status of much of the work but denied part of the motion by the plaintiff, Leslie Klinger.

Most of Sir Arthur Conan Doyle's creation already is in the public domain. This includes anything in the four novels and 46 short stories published prior to 1923. Conan Doyle imitators may legally create anything derived from that substantial stash. Thus such prize cultural artefacts as Sherlock Holmes and the Alien Abduction, Sherlock Holmes and the Mutilated Cattle and the Guy Ritchie movies.

Ten Holmes short stories, however, were published after 1923, the public domain threshold pinpointed by Melville Nimmer in his authoritative Nimmer on Copyright. Details from the last ten stories could still be subject to copyright claims by Conan Doyle's descendants, Judge Rubén Castillo ruled on Monday, in a decision that went unnoticed until Friday.

In protecting the last ten stories, however, Castillo reinforced access to the rest of the Holmes oeuvre. Castillo rejected an argument by the Conan Doyle estate that some aspects of pre-1923 Holmes were not plainly in the public domain because the stories and characters were "continually developed" through the final ten stories, which will not entirely enter the public domain until 2022.

Castillo cited a 1989 case in which CBS corporation tried to stop a Broadway adaptation featuring Amos 'n' Andy, the central characters of a wildly popular radio programme that debuted in 1928. The characters were found to be in the public domain, but "increments of expression" that further "delineated the characters and story" were found to be subject to copyright protections.

The ruling applies only to the US. The entire Sherlock Holmes canon has been in the public domain in Britain since the end of 2000.

Klinger, the plaintiff, hailed the ruling in a statement issued Friday. “Sherlock Holmes belongs to the world,” he said. “This ruling clearly establishes that. Whether it’s a reimagining in modern dress (like the BBC’s Sherlock or CBS-TV’s Elementary), vigorous interpretations like the Warner Bros fine Sherlock Holmes films, or new stories by countless authors inspired by the characters, people want to celebrate Holmes and Watson. Now they can do so without fear of suppression by Conan Doyle’s heirs.”

Klinger sought a declaration in the case after his publisher, Pegasus, delayed his book's release pursuant to a legal threat by the Conan Doyle estate.

Sir Arthur Conan Doyle, who died in 1930, introduced Holmes in A Study in Scarlet, which was first published in Beeton's Christmas Annual in 1887 and released in the United States in 1890.