def sort_standard_chrs(chrom):
    """
    Returns the value on which the
    standard chromosomes can be sorted.

    Parameters
    ----------
    chrom : str
        The chromosome

    Returns
    -------
    int
        The value on which to sort

    """
    chrom = chrom[3:]
    if chrom.isdigit():
        return int(chrom)
    if chrom == 'X':
        return 23
    elif chrom == 'Y':
        return 24
    elif chrom == 'M':
        return 25
    else:
        return 26
