The Secret Grade is a hidden grading mechanism in the TGM series achieved by creating a ">" shape while leaving holes in the stack. When blocking out, the game evaluates the blocks in the field, checking if a zigzag pattern is created. The secret grades are 9 to 1 for the first nine rows of the pattern completed, S1 to S9 for 10-18 rows, and GM for all 19 rows.
Creating the correct holes in the stack is easier to accomplish when a "sawtooth" or "stairs" structure is maintained in the direction of the diagonal you are creating. By maintaining this structure, holes can be covered with others to finish holes and create the foundation for new ones. S, Z, and T are very useful for creating the sawtooth structure or covering holes, although any piece can be used to create useful structures.
Line clears increment the level counter and can prevent progress. For these reasons, line clears are generally avoided. However, line clears can be useful or necessary in a variety of situations. Line clears are required when creating the first holes at either wall. If a line clear is necessary, holes can be built on top of the lines to be cleared and "dropped" into place.
Maintaining an even spread of pieces throughout the stack is very important. Uneven stacking can hinder the completion of holes and reduce placement choices near the top of the stack. Placement choices are limited near the top of the stack, so care should be taken to maintain an even stack without obstructions. If it appears that the stack is growing unstable, the best strategy is to clear lines or backtrack until a stable approach can be made.
Reaching the right wall is like starting over again; a line clear structure is needed to create the first few holes. Structures going to the right wall can be reused when returning to the left, but the pieces used must be flipped. For example, structures that used L before reaching the wall now use J on the return trip.
Starting the structure can be efficiently started with any piece except O. At least one line clear is required when using any other piece, but using O to create the first hole necessitates two.
Starting line clears can also be used in the body of the building process. Try to use structures like these whenever a line clear appears to be necessary.
Other Line Clears
Some line clears can be useful in creating holes, but have little or no use as starting structures.
In the case that you have an open imperial cross or T shape, fill it with S/Z finish a hole and start another.
Cover holes with S/Z to create the foundation for new holes.
Place T next to this foundation to complete the hole and start a new one.
Keep an eye out for ways to create covers from the other side when you get pieces in a different order.
Covering with I can also be effective, but be wary when using L, J, or O; covering with these pieces can create unwanted hanging points or fill the space where you intend to form a hole.
Long pieces such as L, J, and I are effective at reaching into open spaces and finishing holes from the side. Fill open long crosses with L and J as you would fill open imperial crosses with S and Z. I can be used to complete holes in longer openings in a similar fashion.
Hanging points are structures that impede the direct creation of holes; pieces must instead be placed in so they are "hanging" from this structure in order to create a hole. Using O, L, or J to cover a hole have the highest potential for creating hanging points. A line clear may be necessary to continue, but sometimes "hanging" structures can be useful in quickly creating multiple holes with few pieces.
If the structure is 3 units tall, only L/J can be hung from the structure and complete a hole, making a line clear the best strategy.
If the structure is 2 units tall, any non-line piece can be hung from the structure to complete a hole. A few pieces will maintain the 2 unit structure, while a few will necessitate a line clear. T pieces can successfully end the hanging structure without requiring a line clear.
In this diagram, an O piece starts the structure and J and S maintain it. Z and L could have been used to end the structure by clearing a line, but T pieces cap off the structure without a clear.
There are very few ways to complete the pattern. The 19th hole must be covered in order to get the secret GM grade. For this reason, the successful completion of the pattern is dependent on receiving specific pieces depending on the current state of the stack.
The safest way to finish the pattern is to cover the sawtooth at the top with a Z, simultaneously finishing holes 18 and 19. After placing the Z, you can then proceed to fill the rest of row 19.
Riskier methods will depend on receiving one or more I blocks precisely when they are needed. An I block can spawn and move freely in row 20, enabling it to be used as a cover for the incomplete pattern.
An even riskier method involves placing a T flat over hole 18, clearing row 19, and covering hole 19 with a J.
Keep in mind that the piece that causes the game to end can also complete the pattern.
When making considerations for a potential GM solution, it may be advantageous to place L, J, and I blocks vertically on the right side of the field as you approach the top of the field. Several vertical placements of these pieces will work in parallel to fill multiple rows while leaving the spawn area open for new pieces. On the other hand, horizontal placements in the center or on the left may reduce mobility or occupy part of the spawn area.
Here are a few examples of viable finishes:
These setups require a notch on the left side for the T. You can use a vertical I-block as a spacer to use this setup one row lower than normal.
Clearing the center row of vertical L "converts" it to the shape of the T in the notched setups. One must take care to ensure that the pieces used to clear the line do not interfere with forming the hole for that row.