Upon first glance, the chapter numbers of Mark Haddon's The Curious Incident of the Dog in the Night-Time don't appear to follow any rhyme or reason. From the very start, the numbers are slightly askew, as the first chapter is numbered "2" rather than "1." Next, the number "4" is skipped as a chapter number. In chapter "19," which is actually the eighth chapter of the book, the narrator, Christopher John Francis Boone, explains the rationale for this choice:
Chapters in books are usually given the cardinal numbers 1, 2, 3, 4, 5, 6 and so on. But I have decided to give my chapters prime numbers 2, 3, 5, 7, 11, 13 and so on because I like prime numbers.
A phenomenal understanding of prime numbers is one of the first qualities that we are introduced to within the narrator. He mentions this part of himself at the very start of chapter 3:
My name is Christopher John Francis Boone. I know all the countries of the world and their capital cities and every prime number up to 7,057.
Through Boone, Haddon is attempting to replicate the mind and narration style of a character on the autism spectrum. More specifically, Christopher has Asperger's syndrome. This is the reason for Boone's focus on certain concepts like prime numbers and choice to specifically assign them to "his" chapters.