To remove the split diagonally,
Select the cell
Go to Edit
Choose Clear
Choose All

To insert the split
Check to find out if the are border split. If Yes,
GO to Format, Cell, Border Tab,
Select Outline
Click on diagonal border.

"Ann Reed" wrote:

I have a document that came to me with a cell that is split diagonally. You
can write in the lower side of the diagonal and on the upper side of the
diagonal. I do not know how to create this split within the cell and I do not
know how to remove it.
--
Ann Reed