Equilateral triangles and squares are relatively simple, regardless of size;

In the case of an equilateral triangle, the three sides of the inner and outer triangles are parallel to each other, so the three angles of the inner and outer triangles are equal, which proves that the inner and outer triangles are similar;

Only rectangular. Because the length and width of the inner rectangle is the length and width of the outer rectangle minus the same lace width, the length-width ratio changes, so it is not a similar figure. In the most extreme case, if the width of the lace is equal to the width of the outer rectangle, the inner rectangle will become a line segment with the same length (length-width) as the outer rectangle.