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based on an earlier version of the Wolfram Language.
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Cells and Cell Groups

Every Mathematica notebook consists of a sequence of cells, normally indicated by brackets at the right. You can arrange cells in groups to organize your document into sections and subsections.

A group of cells can be closed so that only the head cell is visible. To open a cell group, double-click the bracket of the head cell. Double-click again to close the group.

Here is a notebook with closed cell groups. The arrow at the bottom of a cell bracket indicates a closed group.

Here is the same notebook with cell groups open.