|This article does not cite any references or sources. (July 2011)|
In a graphical user interface, an accordion is a vertically stacked list of items (e.g. labels or thumbnails). Each item can be "expanded" or "stretched" to reveal the content associated with that item. There can be zero or more items expanded at a time, depending on the configuration.
The term stems from the musical accordion in which sections of the bellows can be expanded by pulling outward.
A common example of a GUI accordion is the Show/Hide operation of a box region, but extended to have multiple sections in a list.
Developer definition 
Several buttons or labels are stacked upon one another. At most one of them can be "active". When a button is active the space below the button is used to display a paned window. The pane is usually constrained by the width of labels. When opened it shifts labels under the clicked label down according to the height of that window. Only one button or pane combination can be active at any one time; when a button is selected any other active panes cease to be active and are hidden. The active pane may have scrollbars.
- Brings windows together which ought to have some relation to each other.
- One window available at a time to reduce information "overload" only one window is "opened".
- Unavailable windows are "shortcutted" / shaded to make choice faster. Chat program Google Talk rewrites window labels to indicate important states like "someone is writing" ...
- All windows stacked on each other together - see Fitts's law for further information about it.
User definition 
Several windows are stacked on each other. All of them are "shaded", so only their captions are visible. If one of them is clicked, to make it active, it is "unshaded" or "maximized". Other windows in accordion are displaced around top or bottom edge. Only one is expanded at a time.
A common example using a GUI accordion is the Show/Hide operation of a box region, but extended to have multiple sections in a list.
SlideVerse is an accordion interface providing access to web contents.
The list view of Google Reader also features this.
Apple has some roll-over accordions. For example (as of December 2008), the left column of the page includes three categories that expand on roll-over: "All Downloads", "Top Apple Downloads", and "Top Downloads".
- jQuery UI accordion widget
- mootools Tutorial (where the effect is called sliding shelf) on MONFX
- Accordion Examples
- Accordion Interface Demo of an accordion script