def _scalar_or_pair(type_, **rules):
    """Expand scalar type to also allow a two-element list of the former.

    Parameters
    ----------
    type_ : str
        The type string.
    **rules
        Additional rules to include.

    Returns
    -------
    composite_type : dict
        A dictionary that allows a scalar or two-element list of the former.
    """
    return {'type': [type_, 'list'], 'schema': {'type': type_, **rules},
        'minlength': 2, 'maxlength': 2, **rules}
