The AccordionExtender control is similar to the CollapsiblePanelExtender control. It allows you to group multiple collapsible panels in a single control. At the same time, it also manages the collapsed and expanded state of each panel; therefore, expanding one panel at a time. In other words, the AccordionExtender control does not support expanding two or more panels simultaneously. Instead, the header templates of all the panels are always visible so that you can click on any of them to display the hidden contents. By default, the AccordionExtender control opens with one panel as expanded.