Panels are individual images containing a segment of action, often surrounded by a border. Prime moments in a narrative are broken down into panels via a process called encapsulation. The reader puts the pieces together via the process of closure by using background knowledge and an understanding of panel relations to combine panels mentally into events. The size, shape, and arrangement of panels each affect the timing and pacing of the narrative. The contents of a panel may be asynchronous, with events depicted in the same image not necessarily occurring at the same time.