def proper_ptgrp(cryst_ptgrp):
    """
    Returns the proper point group corresponding to a crystallographic point
    group

    Parameters
    ----------------
    cryst_ptgrp: string
    Crystallogrphic point group in Schoenflies notation

    Returns
    ----------
    proper_ptgrp: string
    Proper point group in Schoenflies notation
    """
    if cryst_ptgrp in ['D3', 'D3d']:
        proper_ptgrp = 'D3'
    if cryst_ptgrp in ['D4', 'D4h']:
        proper_ptgrp = 'D4'
    if cryst_ptgrp in ['D6', 'D6h']:
        proper_ptgrp = 'D6'
    if cryst_ptgrp in ['O', 'Oh']:
        proper_ptgrp = 'O'
    return proper_ptgrp
