def str_to_pos(chess_notation):
    """converts from chess notation to co-ordinates
    >>> str_to_pos("A3")
    (0, 5)
    >>> str_to_pos("B6")
    (1, 2)
    """
    letter = chess_notation[0]
    x = ord(letter) - ord("A")
    y = 8 - int(chess_notation[1])
    return x, y