Excel’s fill handle is a small rectangular control that appears whenever a cell or range of cells are selected.
When the cursor is moved over the fill handle, it turns into a cross symbol, which can be dragged to fill in more cells.
The behavior of the fill handle varies depending on the content of the cell or range selected:
When an adjacent column contains values, double-clicking the fill handle will extend values down the worksheet to match, following the behaviors described above.
