def _quads_to_triangles(quads):
    """Convert a list of quads to a list of triangles.

    :param quads: The list of quads.
    :type quads: list of 4-tuples
    :returns: List of triangles.
    :rtype: list of 3-tuples

    """
    triangles = []
    for quad in quads:
        triangles.append((quad[0], quad[1], quad[2]))
        triangles.append((quad[0], quad[2], quad[3]))
    return triangles
