The column containing the cell has to be defined using the Column.Merge property, defining that the cell may merge into the next column's empty cell. The next column has to be defined using the Column.MergeInto property, indicating that it can be "merged into". The cell in the next column must be empty, i.e. it has no cell text or cell graphic.
Column headers and column footers also use the settings found in the Column.Merge and Column.MergeInto properties. If a column header or column footer is empty and the previous column has been adequately defined using the Column.Merge property, the column headers or column footers merge. If the columns are resizable, a small resizing handle is shown in the combined column header or column footer.