In Agatha Christie's And Then There Were None, a murder occurs among a group of strangers in a house on an isolated island.
The closed circle of suspects is a common element of detective fiction, and the subgenre that employs it can be referred to as the closed circle mystery.[1][2][3] Less precisely, this subgenre – works with the closed circle literary device – is simply known as the "classic", "traditional" or "cozy" detective fiction.[4][5]
It refers to a situation in which for a given crime (usually a murder), there is a quickly established, limited number of suspects, each with credible means, motive, and opportunity.[1][2][6][7] In other words, it is known that the criminal is one of the people present at or nearby the scene, and the crime could not have been committed by some outsider.[3][8] The detective has to solve the crime, figuring out the criminal from this pool of suspects, rather than searching for an entirely unknown perpetrator.[1][3]
Those early closed circle mysteries preferred a common setting: a British country house.[3][8][9][11][13] The country house was a common enough element that closed circle mysteries set in such a location are sometimes known as "country house mysteries".[14] The persons involved were also commonly part of the upper class, generally the landed gentry.[3][7][11][13] Other settings than the country house are possible, such as ships, trains, islands, and so on.[8][10] The requirements for the setup of the mystery enforce certain limitations on the genre. Certain settings are frequently represented in the genre, typically involving upper-class characters to whose properties outsiders have limited access.[13][15] The numbers of suspects vary, from a group as small as four or five, to all the passengers of a train, coach or wagon.[8]
After the Second World War, the closed circle mystery became less common as other types of crime novels rose to prominence;[16] nonetheless, writers such as Rex Stout, Lucille Kallen, Cyril Hare, Jonathan Gash, and Simon Brett have employed the device in their fiction.[14]
While the closed circle is a common device in literary fiction, it is a much less common occurrence in actual criminal investigations.[3]