What is duology?

A duology is a series consisting of two books. Similar to a trilogy, it is a sub-genre of series. It is often a self-contained story, with the first book setting up the characters and plot, and the second book resolving the conflict. Unlike trilogies which are quite common, duologies are less popular. Some popular examples of duologies include "Six of Crows" and "Crooked Kingdom" by Leigh Bardugo and "The Merciless" and "The Merciless II" by Danielle Vega. Duologies are becoming increasingly popular in the young adult and fantasy genres.