Why were Victorian novels often first published in three volumes?

In the Victorian era, the three-volume format—often referred to as the "three-decker"—was the standard mode for the first publication of novels. This format was specifically designed to cater to the business models of lending libraries, which were the primary purchasers and distributors of new fiction at the time.1

These circulating libraries, such as the influential Mudie’s Circulating Library, held significant power over the publishing industry. By purchasing large quantities of three-volume sets, these institutions provided a steady, reliable market for publishers. Because these libraries could circulate the three separate volumes to different readers simultaneously, the format maximized the utility and profitability of their collections.213

While this "three-decker" system dominated for much of the 19th century, it faced competition starting in the 1830s from the rise of serial publication in magazines and monthly parts. The three-volume standard eventually declined and was replaced by the modern single-volume format during the 1890s.1