def letter_only(guess, abc):
    """
    Checks if the player's guess is a single ASCII letter only.

    :param abc: the alphabet used in the proverbs
    :type abc: str
    :param guess: the player's guess
    :type guess: str
    :return: True | False
    :rtype: bool
    """
    if len(guess) == 1 and guess.upper() in abc:
        return True
    return False