Empty marks in LITS Puzzle are reasoning notes only and have no effect on completion, validation, or scoring. The Enter key places or removes a small mark on an unshaded square, but the validator never reads those marks when checking whether the board is solved. What actually controls completion is the set of shaded cells you commit with the Space key: each bold region must contain exactly four shaded cells forming a recognizable L, I, T, or S tetromino, the global shaded network must remain connected across the region border, no fully shaded 2×2 block may appear, and touching tetrominoes of the same normalized type are rejected even when one is rotated or reflected. Score is calculated solely from shade actions, not from marks. Because empty marks sit on unshaded cells and never participate in shape recognition, connectivity checks, or the 2×2 scan, you can place or clear them at any time without changing the verdict the engine returns when you finish shading the eight disclosed cells.
When you're halfway through a LITS board and you've narrowed a region down to two candidate shapes, the Enter key lets you pencil-mark the cells you've decided to skip. Many players worry those marks will accidentally count against them when they finish. They won't. Empty marks are pure scratchpad content: visible to you, invisible to the checker. This makes them safe to use freely as a planning layer while you build the tetrominoes with Space.
What Empty Marks Are and How They Behave
An empty mark is a small annotation you can place on any cell that is currently unshaded. You add one by pressing Enter on the highlighted cell, and you remove it by pressing Enter again on the same cell. Mouse and touch users trigger the same mark through the cell's context-menu gesture. The mark stays on the square until you change it yourself, restart the board, or trigger the boss-key cover. The mark never changes to a shaded cell on its own, no matter how many times you press Enter, and pressing Space on a marked cell removes the mark and applies a shade in one step.
The board treats a marked cell exactly the same as a plain unshaded cell. Validation, connectivity, the 2×2 scan, and the same-type touching rule all see only the shaded cells, never the marks. If you ever want to see exactly what the validator sees, just close your eyes to the marks and read the shaded squares only — that is the complete input the engine considers.
What Actually Determines Completion in LITS Puzzle
Four interacting rules decide whether the board is complete, and empty marks sit outside every one of them. The first rule is the regional shape rule: each bold-bordered region must contain exactly four shaded cells, and those four cells must form one of the four permitted tetromino types — L, I, T, or S. The O tetromino is excluded because it is a filled 2×2 square, which would directly violate the second rule.
The second rule is the global 2×2 prohibition: no fully shaded 2×2 block may appear anywhere on the board. The third rule is the same-type touching rule: any two tetrominoes that share an edge must be of different normalized types. Rotations and reflections are not treated as distinct types, so a vertical I and a horizontal I count as the same type and are rejected if they touch. The fourth rule is the connectivity rule: all shaded cells on the whole board must form one orthogonally connected network, even when they belong to different regions. Region borders are not walls; they are ordinary grid lines, and the connectivity check crosses them with a breadth-first traversal.
These four rules are checked independently. A region with more than four shaded cells is rejected outright. Four cells that do not form one of the four permitted shapes are rejected. A filled 2×2 anywhere is rejected immediately. Once both regions pass, the checker compares touching shapes and rejects any pair that normalizes to the same type. Only after all four checks pass is the board counted as complete.
How to Use Empty Marks Without Disturbing Completion
Because empty marks are isolated from the validator, you can use them as a planning layer while you build the solution. The following routine keeps your notes useful and never puts the completion check at risk.
- Open the LITS Puzzle board and identify the two bold regions: Region A in the leftmost two columns and Region B in the remaining three columns.
- Move with the arrow keys to a cell you are confident should remain unshaded, then press Enter to leave a small empty mark.
- Repeat for each cell you intend to skip, using Enter as your scratchpad for candidates you've eliminated.
- Move to a cell you want to commit and press Space to shade it; press Space again to remove a shade.
- After shading four cells in a region, mentally check that the shape is one of L, I, T, or S, that no two of the four cells form a filled 2×2 block, and that the four cells touch the other region through at least one shared edge.
- When both regions are shaded, confirm that touching tetrominoes have different normalized types and that all eight shaded cells are reachable from any other shaded cell by moving up, down, left, or right.
- If a mark no longer helps your reasoning, press Enter again on that cell to clear it. Clearing marks is optional and never changes the final score.
Pressing Enter accidentally does not cost you any points, and forgetting to clear old marks will not lower your score. The score engine subtracts ten points for every shade action it counts, not for any mark you leave behind. If you ever want to start fresh, the restart control wipes both shaded cells and empty marks and resets the score to 1,000.
Empty Marks vs. Shaded Cells at a Glance
The table below summarizes how each control behaves in the four systems the validator runs against your board. Empty marks sit in the "no" column for every check, which is why they cannot make or break completion.
| Action | Affects completion? | Counts toward score? | Read by shape check? | Read by 2×2 scan? |
|---|---|---|---|---|
| Press Space to shade | Yes | Yes (−10 per shade action) | Yes | Yes |
| Press Space to unshade | Yes (changes shape) | Reduces shaded count | Yes (after unshade) | Yes (after unshade) |
| Press Enter to add an empty mark | No | No | No | No |
| Press Enter to remove an empty mark | No | No | No | No |
| Press Arrow keys to move | No | No | No | No |
That breakdown shows why experienced solvers treat Enter as a free scratchpad: it costs nothing, changes nothing, and disappears the moment the board is read for completion.
Score, Validation, and Empty Marks
The score engine runs a single formula on the shade actions only: max(0, 1,000 − 10 × shade actions). The disclosed 8-shade solution therefore scores 1,000 − 10 × 8 = 920. Marks and notes are not counted as shade actions, so a player who marks fifty cells and then shades eight will still finish on 920. Likewise, a player who shades fewer than eight times and then realizes their shape is wrong will not lose any points to the marks they left behind.
Validation runs in a fixed order. The engine first groups shaded cells by region using the bold borders, then checks the four-cell count per region, then normalizes the four cells across all eight rotations and reflections and compares against the L, I, T, and S bases. Every 2×2 window is scanned next. Cross-region touching types are compared with the same normalization table, so a vertical I in Region A and a horizontal I in Region B would be rejected because they normalize to the same type. Finally, a breadth-first traversal checks that all shaded cells are reachable from each other. Empty marks never enter this pipeline.
When Empty Marks Are Cleared and Where They Aren't
Restart clears everything: shades, marks, the highlighted cell, and the score all return to their starting state. The boss-key cover, triggered by a double press of Escape, hides the board behind a neutral spreadsheet view; pressing Escape twice more brings the same board back with your marks and shades intact. There is no auto-clear timer, so a long thinking pause will not silently erase your notes. Best score is the only state that persists across sessions, and it is stored locally without any marks attached.
If you want to practice without leaving marks behind, you can simply skip the Enter key and rely on shading and unshading with Space. The board will still validate the same way. If you find empty marks helpful for keeping track of rejected cells during a hard board, use them freely — they cost nothing, change nothing, and disappear the moment you restart.
For a deeper dive into the L, I, T, and S shape definitions, the original Nikoli rules remain the canonical reference. Nikoli publishes the four-shape rule set and the same-type touching rule in its LITS guide, and Puzzle LITS maintains an independent cross-check that verifies all four constraints, including the 2×2 prohibition and the connectivity requirement.