Enlarge each side of the H pattern √18 = 3√2 times as the original, you'll get 18 times of area H. Put a black hole ( enlarge the H pattern twice ) anywhere on the fixed frame, and then fill in the other area with 18 ProTangrams.
If the pattern left resembles the black hole in the center, it is called " Tetratans' Ring ".
A Tetratans' Ring with the pattern H in the center is also called a Rectangle Ring or H ring.
A Tetratans' Ring with the pattern X in the center is also called a Square Ring or X ring. There are more solutions and relationships list available in Note 2.SR ( maxside.htm ) .
Put the 8-tan' black hole on the frame ( as close to the center as possible ), and then fill in the other area with the 18 ProTangrams. If the pattern left happens to resemble the black hole, that means you probably find the best solutions.
All the Tetratans can be solved except the pattern I ? Some of the black holes can be solved by removing them on the verge of the frame. Not bad the pattern S can be solved by its mirror.
All the Tetratans ( 14 pieces in total ) can be solved. The most interesting solution among them is the pattern H, whose solution consists of the symmetry of patterns.
ref: The Poly Pages by Andrew Clarke / Problems / Similar Hole Problems
Rotate the 8-Tans' black hole ( 14 pieces in total ) 90 degree either clockwise or counterclockwise and put it in the center of the fixed frame. All the 14 resemble patterns can be solved.