An accordion is a stacked list of item references (e.g. labels or thumbnails), where each item can be "expanded" or "stretched" to reveal more content associated with that item. Typically only one item can be expanded at a time, and expanding a second item causes the last opened item to be closed.
What is the best way to indicate that it's possible to open and close an element accordion-style? Arrow pointing left/arrow pointing down Arrow pointing down/arrow pointing up Plus sign/minus sign … ...