def _box(bl, ur):
    """(x, y) coordinates for the 4 lines making up a rectangular box.

    Parameters
    ==========
    bl : float
        The bottom left corner of the box
    ur : float
        The upper right corner of the box

    Returns
    =======
    coords : 2-tuple
        The first and second elements of the tuple are the x and y coordinates
        of the box.
    """
    xl, xr = bl[0], ur[0]
    yb, yt = bl[1], ur[1]
    box_x = [xl, xr,
             xr, xr,
             xr, xl,
             xl, xl]
    box_y = [yb, yb,
             yb, yt,
             yt, yt,
             yt, yb]
    return (box_x, box_y)