A valid LITS solution shades exactly four cells in each bold-bordered region so those cells form one of four tetromino shapes — L, I, T, or S — while every shaded cell on the whole board stays connected into one orthogonal network, no filled 2×2 square appears anywhere, and matching shape types never touch by an edge. The mistakes solvers make most often come from forgetting one of those layered rules mid-game, and each one can quietly destroy an otherwise correct board. The fastest way to dodge those mistakes is to play on a board that re-checks every rule after every move, so a misplaced cell is flagged the instant it breaks a constraint. The browser LITS Puzzle tool groups shaded cells by region, normalizes them against the four tetromino bases, scans every 2×2 window, breadth-first traverses the full shaded set, and rejects touching shapes that share the same normalized type. That means instead of discovering a fatal 2×2 block at the end, you see it the moment the fourth corner is shaded, which is the single most useful habit a beginner can build when learning how to avoid common mistakes in LITS puzzle play.
The Four Rules a Valid LITS Board Must Satisfy
According to Nikoli's original LITS rules, a solved board has to satisfy four constraints at the same time, and forgetting any one of them is what creates the most common mistakes. Here is what each rule actually means in practice.
- One tetromino per region. Each bold-bordered region must contain exactly four shaded cells whose relative positions match an L, I, T, or S tetromino. Rotations and reflections count as the same type, so a sideways L is still an L. The O tetromino is excluded by the next rule.
- No filled 2×2 block anywhere on the board. Even if your L and T are individually correct, four shaded cells that form a solid 2×2 square break the global rule. This is the rule beginners violate most often by accident.
- All shaded cells must connect. When you treat every shaded cell as a node on an orthogonal grid, the whole set must be reachable from any starting shaded cell by walking up, down, left, or right. Two perfectly valid tetrominoes that never touch are an invalid solution.
- Touching shapes must be different types. Two tetrominoes that share even a single edge must belong to different shape categories. An L next to an L is illegal, even if the L is rotated, reflected, or shifted into a different orientation.
The order of the rules matters less than the fact that all four must hold simultaneously. The LITS Puzzle tool checks every one of them after each shade action, so the moment any rule breaks, the board stops counting toward completion.
Five Frequent Mistakes and What the Validator Rejects
The fastest way to avoid mistakes in LITS is to know what each one looks like before you shade the cell that triggers it. The table below pairs each mistake with the constraint it breaks and the specific check the tool runs to catch it.
| Mistake | Constraint it breaks | How the validator catches it |
|---|---|---|
| Accidental 2×2 block | Global no-square rule | Scans every 2×2 window after each shade action |
| Two separated tetrominoes | Connectivity rule | Breadth-first traversal of all shaded cells |
| Same shape type touching edge-to-edge | Touching-type rule | Normalizes both shapes and compares them to the I, L, T, S bases |
| More than four cells in one region | One-tetromino-per-region rule | Counts cells grouped by the two declared regions |
| Four cells that do not form L, I, T, or S | One-tetromino-per-region rule | Normalizes the four-cell set against all rotation and reflection variants |
Notice how the touching-type rule depends on normalization. If you place a vertical I in Region A and a horizontal I in Region B and let them touch by an edge, the validator still classifies both as type I and rejects the board. The check ignores rotation and reflection on purpose, which is exactly what catches the second-most-common mistake after the 2×2 slip.
How to Play LITS Puzzle Without Triggering the Common Mistakes
- Open LITS Puzzle in any modern browser. The board shows two bold-bordered vertical regions across a four by five grid of cells, plus a status row and a starting score of 1,000.
- Move the selector with the arrow keys until the highlighted cell is the one you want to shade. Press Space to shade it, or press Space again to unshade. Mouse users can click a cell to toggle its shade.
- Press Enter on an unshaded cell to drop an optional empty mark — a small reasoning note that helps you remember why you left that cell blank. Mouse users can right-click or use the context-menu gesture. Empty marks never affect completion or score.
- Plan one tetromino per region before you start shading. A vertical I needs four cells stacked in a single column. A horizontal I needs four cells in a single row. An L needs three in a line plus one perpendicular foot. A T needs a three-cell bar with a centered stem. An S needs two cells, a perpendicular step, and one more cell continuing the offset.
- Shade one region first, then check that no four shaded cells in that region accidentally form a 2×2 square. If they do, undo by re-pressing Space on one cell before moving on.
- Move to the second region and place a tetromino whose type is different from the first region's type. If both regions feel forced into I shapes, shift one into a T or an L so the touching pair stays legal by type.
- Make sure the two tetrominoes share at least one edge. On this fixed board, the disclosed solution touches through the edge between cell 6 and cell 7, which is what turns two shapes into one connected network.
- Keep shading until both regions hold exactly four cells, every cell is reachable from every other shaded cell, and the two touching shapes belong to different tetromino types. The status display will mark the board solved.
- If a mistake has already cost you shade actions, press Restart to clear shades and notes and restore the score to 1,000. The best score from any previous run stays in localStorage only and never leaves the browser.
Worked Example: Building the Disclosed Eight-Cell Solution
The board's two bold regions are split as Region A — the first two columns — and Region B — the remaining three columns. The disclosed solution places a vertical I tetromino in Region A at the zero-based cells 1, 6, 11, and 16, which is the second column straight down the board. Region B receives a T tetromino at cells 7, 8, 9, and 13: a horizontal bar of three cells across the top of Region B with one centered cell hanging straight down from the middle of the bar.
Counting shade actions in order: shade cell 1, cell 6, cell 11, and cell 16 — that is four actions for Region A. Shade cell 7, cell 8, cell 9, and cell 13 — that is four more actions for Region B. The total is 8 shade actions, which costs 80 points from the starting 1,000 and leaves a final score of 920. The formula the tool uses is max(0, 1000 − 10 × shade actions), so any mistake that forces an extra shade or an unshade directly eats into your score.
Walk through the four rule checks on this completed board:
- One tetromino per region. Region A's four cells line up in a single column, which matches the vertical I base after normalization. Region B's three-in-a-row plus a centered stem matches the T base. Both pass.
- No 2×2 block. The completed board contains no four shaded cells that form a solid square. The T's stem at cell 13 has unshaded neighbors on three sides, so no 2×2 window fills.
- Connectivity. Cell 6 in Region A and cell 7 in Region B sit side by side, so every shaded cell is reachable from every other shaded cell through a single orthogonal walk. The breadth-first traversal confirms one connected network.
- Touching shapes differ. The only adjacent pair is the I in Region A and the T in Region B. After normalization, those types differ, so the touching rule passes.
Any misplaced shade action that breaks one of the four constraints is the kind of mistake the validator catches before you ever see the 920 score, and the kind of mistake the tool is built to keep you from carrying to the end of the board.
Other Logic Puzzles With Layered Rule Sets
LITS is one of several Nikoli-style puzzles where a small grid hides a layered rule set, and the habit of re-checking every rule after every move transfers directly to other shape-recognition games. If you want to compare the rule wording against the publisher's definition while you play, the Nikoli LITS rules page lists the same four constraints in the original phrasing.
For a related reader question on this exact tool, the companion guide Do Empty Marks Affect Completion in LITS Puzzle? confirms that notes never change a board's validity, which is useful when you want scratch space without spending shade actions on it. Reading both together covers the full set of beginner pitfalls in a single sitting.
Related reading: Logic Gates Puzzle Online: Build Five TRUE Circuits.