def load_overlaplcwp_timegrid(self, blockid=0, key=("ov", "ovkin", "ovpot")):
    r"""Load the timegrid corresponding to the overlap matrices specified.

    :param blockid: The ID of the data block to operate on.
    :param key: Specify which overlap matrices to load. All are independent.
    :type key: Tuple of valid identifier strings that are ``ov``, ``ovkin`` and ``ovpot``.
               Default is ``("ov", "ovkin", "ovpot")``.
    :return: A list of :py:class:`ndarray` each having one column.
    """
    tg = []
    for item in key:
        if item == "ov":
            pathtg = "/" + self._prefixb + str(blockid) + "/overlaplcwp/timegrid"
            tg.append(self._srf[pathtg][:])
        elif item == "ovkin":
            pathtg = "/" + self._prefixb + str(blockid) + "/overlaplcwp/timegridkin"
            tg.append(self._srf[pathtg][:])
        elif item == "ovpot":
            pathtg = "/" + self._prefixb + str(blockid) + "/overlaplcwp/timegridpot"
            tg.append(self._srf[pathtg][:])
        else:
            raise ValueError("Unknown key value {}".format(item))

    if len(tg) == 1:
        print(tg)
        return tg[0]
    else:
        return tuple(tg)