Cells are stored in column-major order with y increasing up, allowing [i][j] addressing:
+---+---+---+ | d | e | f | +---+---+---+ | a | b | c | +---+---+---+
The cells property is list of lists: [['a', 'd'], ['b', 'e'], ['c', 'f']]
Thus the cell at (0, 0) is 'a' and (0, 1) is 'd'.