def span_begin_end_coords(span_length1, span_length2=0.0):
    """Calculate the span beginning and end coordinates for spans 1 and 2.
    
    Args:
        span_length1 (float): length of span 1
        span_length2 (float, optional): length of span 2

    Returns:
        span1_begin (float): coordinate location of beginning of span 1
        span1_end (float): coordinate location of end of span 1
        span2_begin (float): coordinate location of beginning of span 2
        span2_end (float): coordinate location of end of span 2

        Parameters are returned as a tuple in the following order:
        (span1_begin, span1_end, span2_begin, span2_end)
    """
    if span_length1 < 0.0:
        raise ValueError("Must enter a positive float for span 1 length.")
    elif span_length2 < 0.0:
        raise ValueError("Must enter a positive float for span 2 length (or"
                            "nothing for a default value of 0.0).")
    else:
        span1_begin = 0.0
        span1_end = round(span_length1,3)
        span2_begin = round(span_length1,3)
        span2_end = round(span_length1 + span_length2,3)
        return span1_begin, span1_end, span2_begin, span2_end