In the title of the John Boyne novel, what pattern were the boy's pyjamas?

The Boy in the Striped Pyjamas is a 2006 Holocaust novel by Irish novelist John Boyne. Unlike the months of planning Boyne devoted to his other books, he said that he wrote the entire first draft of The Boy in the Striped Pyjamas in two and a half days, barely sleeping until he got to the end. As of March 2010, the novel had sold more than five million copies around the world. In both 2007 and 2008, it was the best selling book of the year in Spain, and it has also reached number one on the New York Times bestseller list, as well as in the UK, Ireland, and Australia. The book was adapted in 2008 as a film of the same name.

Bruno is a nine-year-old boy growing up during World War II in Berlin. He lives with his parents, his 12-year-old sister Gretel and maids, one of whom is called Maria. After a visit by Adolf Hitler, Bruno's father is promoted to Commandant, and the family has to move to 'Out-With' because of the orders of "The Fury" (Bruno's naïve interpretation of the word 'Führer'). Bruno is initially upset about moving to Out-With (never identified, but cf. Auschwitz) and leaving his friends, Daniel, Karl, and Martin.