Much of his popularity derives from his ability to allow the reader to participate vicariously in the ongoing adventure. As with many later children's writers, the stories centred on a small core group of characters who form a close knit unit – at St Jim's there was the Terrible Three, at Rookwood the Fistical Four and at Greyfriars, The Famous Five. Such groups, while being closed to other pupils, are implicitly open to readers who are subliminally invited to include themselves amongst their number, thereby establishing their involvement with the story.