Modern spreadsheet applications allow for more than one two-dimensional "sheet," effectively stacking related spreadsheets on top of each other and introducing a third dimension.
In all spreadsheet programs including Microsoft Excel, rows are labeled using numbers (e.g., 1 to 1,048,576). All columns are labeled with letters starting with the letter A and then incrementing by a letter after the final letter Z. For example, after the letter Z, the next column is AA, AB, AC, ..., AZ and then incrementing to BA, BB, BC, etc. all of the way to the last column XFD. When working with a cell, you combine the column with the row. For example, the very first cell is in column A and on row 1, so the cell is labeled as A1.