def rectangle_clip(recta, rectb):
    """
    Return the clipped rectangle of ``recta`` and ``rectb``. If they
    do not intersect, ``None`` is returned.

    >>> rectangle_clip((0, 0, 20, 20), (10, 10, 20, 20))
    (10, 10, 10, 10)
    """
    ax, ay, aw, ah = recta
    bx, by, bw, bh = rectb
    x = max(ax, bx)
    y = max(ay, by)
    w = min(ax + aw, bx + bw) - x
    h = min(ay + ah, by + bh) - y
    if w < 0 or h < 0:
        return None
    return (x, y, w, h)