Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
If ActiveCell.Value = "" Then ActiveCell.FormulaR1C1 = "x": ActiveCell.Offset(1, 0).Range("A1").Select: End
If ActiveCell.Value = "x" Then ActiveCell.FormulaR1C1 = ""
Rem ActiveCell.FormulaR1C1 = "x"
ActiveCell.Offset(1, 0).Range("A1").Select
End Sub