FOUNDATIONS · LESSON 4 OF 5
Your first clean stack
AFTER THIS GUIDEYou’ll be able to keep a 9–0 well open and explain whether your next placement makes the board easier or harder to continue.
Nine columns to build. One way down.
In a 9–0 stack, you build across the nine left columns and leave the rightmost column as a well. A vertical I piece can fill that channel. When four complete rows clear together, that is a Tetris.
“Clean” does not mean perfectly level after every piece. It means you have usable places for the next pieces and have not buried empty cells you still need to reach. Small steps can be useful; a narrow, deep gap that only an I can fill is a more demanding commitment.
Can I still reach the well? Am I covering an empty cell? Where could the next piece go?
WORKED EXAMPLE · O → I
Fix the surface without closing the well.
This is a simplified six-row slice of the bottom of a board. Grey cells are already locked. The active piece is O and the next piece is I. Work through the decisions before looking at the last frame.
Read the landing
Columns eight and nine have a two-cell-deep step, supported from below. The rightmost column is open all the way down. The O can fill the step without touching the well.
Place O on the step
The yellow O fills columns eight and nine. There are no empty cells underneath it. Four rows are now ready except for the rightmost cell of each row.
Send I down the open column
Rotate I vertically and drop it into column ten. It reaches the bottom because nothing blocks the channel. These four rows are full; the picture shows the instant before they clear.
Read what remains
Here, the four cleared rows were the whole stack, so the board is empty. A taller board would leave its upper blocks behind. The aim of this drill is an accessible stack, not a Perfect Clear.
Pause and check: What if you placed the O one column farther right? You would cover the well and trap empty cells beneath the O. The immediate placement fits, but the next I loses its route.
Three tempting mistakes
“It fits, so it must be fine.”
Look below the landing, not just at its outline. A piece can fit on the surface and still seal a hole underneath. Check the empty cells before you commit.
Building a second I-only gap
The main well already asks for an I. A second deep, single-column gap asks for another one. Try to avoid creating that extra dependency just to make the current piece look tidy.
Waiting for a Tetris at any cost
A small clear is a useful choice when it restores access or lowers a difficult stack. For this drill, a playable board matters more than getting every clear in groups of four.
TAKE IT TO THE BOARD
20 pieces. Keep the way down open.
- Open 9–0 mode. Set Gravity to Off and NEXT to 5; leave Auto Play off.
- Before each placement, name a possible landing for the next piece. Keep the rightmost column accessible.
- Play 20 pieces. Take smaller clears when they help you stay low and avoid holes.
- If you bury a gap, pause. Undo the placement and compare one alternative. Then continue.
What counts as progress? You can point to the first placement that closed a route or created a dependency—and explain a better option. A flawless run is not required.
Open 9–0 practiceOpens the mode. Set Gravity and NEXT yourself; your current settings are kept.
Keep these terms nearby
NEXT IN THIS PATH
6–3 and why it matters
Move the well inward and start thinking about T-spin opportunities.
Further reading
The worked example and drill were created for Stacking Lab. For broader stacking principles, see How to Tetris: Stacking. For the basic line-clear rules, see the official Tetris how-to-play page.