Private Sub Workbook_SheetBeforeDoubleClick(ByVal Sh As Object, ByVal Target As Range, Cancel As Boolean)
    If Not Intersect(Target, Range("A1:A19")) Is Nothing Then
        If Len(Trim(Target)) = 0 Then
            Target.Value = "X"
            Cancel = True
        ElseIf UCase(Trim(Target)) = "X" Then
            Target.ClearContents
            Cancel = True
        End If
    End If
End Sub