Keynote

Merge and unmerge cells in Keynote for iCloud

You can merge table cells to combine adjacent cells into a single cell. Unmerging cells that were previously merged retains all the data in the top-left cell.

Select a range of cells you want to combine.
Click Cell at the top of the Format sidebar on the right, then click the Merge button below Merge Cells.
The contents of all cells are combined into one cell.
To separate the cells again, select the cell, click Unmerge, then copy and paste the contents into the individual cells.

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Keynote User Guide for iCloud

Merge and unmerge cells in Keynote for iCloud