Your complete reference for every Akari and Light Up puzzle term you need to know.
Welcome to the GridPaw akari glossary — a comprehensive reference for every term you will encounter while solving Akari (Light Up) puzzles. Whether you are a beginner learning your first akari puzzle or an experienced solver brushing up on advanced techniques, this puzzle glossary covers all the essential akari terms in one place. Bookmark this page and return whenever you come across unfamiliar terminology.
A number cell is a black cell in an Akari puzzle that displays a digit from 0 to 4. This number tells you exactly how many light bulbs (or cats, on GridPaw) are placed in the four orthogonally adjacent cells — up, down, left, and right. Number cells are the primary source of logical constraints in every Akari puzzle. A "0" means none of its neighbors can hold a bulb, while a "4" means all four neighbors must contain one. Mastering number cell logic is the first step to solving any Akari puzzle.
A bulb (also called a cat on GridPaw) is the object you place on white cells to illuminate the grid. Each bulb sends light in four directions — up, down, left, and right — until the light hits a black wall or the grid edge. The goal of every Light Up puzzle is to place bulbs so that every white cell is illuminated. On GridPaw, bulbs are replaced with adorable glowing cats, but the mechanics are identical. Learn more about placing bulbs in our how to play guide.
To light up or illuminate a cell means that at least one bulb's line of sight reaches it. When you place a bulb, it illuminates every white cell in its row and column until blocked by a wall. The win condition of every light up puzzle requires that all white cells are illuminated simultaneously. Cells that are not reached by any bulb remain "dark" and indicate that the puzzle is not yet solved.
An X mark is a player-placed annotation on a white cell to indicate that no bulb should go there. X marks are not part of the official Akari rules — they are a solving aid. Most online Akari implementations, including GridPaw, let you place X marks by right-clicking or long-pressing a cell. Using X marks helps you track elimination logic and avoid accidentally placing bulbs in impossible positions. See our how to play guide for more on using X marks effectively.
A black wall is any dark cell in the grid that blocks light propagation. Black walls come in two types: plain walls (no number) and numbered walls. Plain walls simply block light — they carry no adjacency constraints. Numbered walls, on the other hand, enforce a specific bulb count on their neighbors. Understanding the difference between these wall types is fundamental to solving Akari puzzles. Read the complete Akari rules for details.
Elimination is the process of ruling out impossible bulb placements based on logical deduction. When you determine that a cell cannot contain a bulb — because it would violate a number constraint, create a bulb conflict, or leave another cell permanently dark — you eliminate it. Elimination is the bread and butter of Akari solving: every step forward comes from either placing a bulb or eliminating a position. Advanced solvers use systematic elimination chains to crack hard puzzles. We will cover elimination strategies in detail in our upcoming tips and strategies guide.
Constraint propagation is the foundational solving technique in every Akari puzzle. It works by examining numbered cells and deducing forced placements or eliminations. For example, a "4" cell forces all four neighbors to be bulbs, while a "0" cell eliminates all four neighbors. After each placement, you propagate the consequences — newly placed bulbs illuminate rows and columns, which may trigger further deductions. Constraint propagation is what the GridPaw Akari solver uses as its first pass on every puzzle.
backtracking is an advanced solving technique — and the method computer solvers use when simple deduction is not enough. You pick an undecided cell, assume it holds a bulb, and propagate all consequences. If you reach a contradiction, your assumption was wrong and that cell cannot hold a bulb. If no contradiction appears, you continue until the puzzle is solved. Human solvers rarely need formal backtracking on Easy or Medium puzzles, but Hard puzzles on larger grids may require it. The GridPaw Akari solver uses backtracking as its final resolution step.
Akari (あかり) is the original Japanese name for this logic puzzle, meaning "light." It was created and published by Nikoli in 2001. The name reflects the core mechanic: placing light sources to illuminate every dark cell. Akari is one of Nikoli's most popular puzzle types alongside Sudoku, Kakuro, and Slitherlink. Learn more about the puzzle's history and design on our Akari puzzle page.
A light up puzzle is the English name for Akari. The two terms are completely interchangeable — same rules, same logic, same solving techniques. "Light Up" became the standard English localization when Nikoli's puzzles spread internationally through newspapers and online puzzle sites. Some players prefer "Light Up" because it is self-descriptive, while purists and Japanese puzzle fans stick with "Akari." Either way, you are playing the same game. Explore our full light up puzzle guide for rules, strategies, and history.
Nikoli (ニコリ) is the legendary Japanese puzzle publisher that created Akari, Sudoku, Kakuro, Slitherlink, and dozens of other logic puzzles. Founded in 1980, Nikoli's editorial philosophy is strict: every puzzle must have a unique solution solvable through pure logic alone — no guessing, no trivia, no luck. This philosophy makes Nikoli puzzles the gold standard for logic puzzle quality worldwide. Visit Nikoli's official website to learn more about their puzzle catalog.
A daily challenge is a new Akari puzzle released every day for players to solve. Daily challenges are a popular feature across puzzle sites because they create a solving habit and give players a shared experience to discuss. On GridPaw, the daily Akari challenge offers a fresh puzzle each day with a balanced difficulty level suitable for most solvers. Come back every day to keep your deduction skills sharp and build your solving streak.
Each term below expands into the full deep dive from our glossary — pros and trade-offs, a step-by-step guide, and the questions players actually ask. Tap any term to unfold it.
Number Cell — A number cell is a black cell in an Akari puzzle that displays a digit from 0 to 4. This number tells you exactly how many light bulbs (or cats, on GridPaw) are placed in the four orthogonally adjacent cells — up, down, left, and right.
Number cells are the primary source of logical constraints in every Akari puzzle. A "0" means none of its neighbors can hold a bulb. A "4" means all four neighbors must contain one. A "2" means exactly two of the four adjacent cells have bulbs. The number never counts diagonal neighbors — only the four orthogonal directions.
A "0" means none of its four adjacent cells can contain a light bulb. It immediately eliminates four positions, which often triggers a chain of further deductions.
No. Number cells only count orthogonal neighbors — up, down, left, and right. Diagonal cells are not counted.
No. Number cells are fixed clues placed by the puzzle designer. They are always correct. If your bulbs conflict with a number cell, the error is in your placement.
Bulb / Cat — A bulb (also called a cat on GridPaw) is the object you place on white cells to illuminate the grid. Each bulb sends light in four directions — up, down, left, and right — until the light hits a black wall or the grid edge.
When you place a bulb on a white cell, it illuminates every white cell in its row and column until blocked by a wall. The goal of every Light Up puzzle is to place bulbs so that every white cell is illuminated. On GridPaw, bulbs are replaced with adorable glowing cats, but the mechanics are identical.
There is no mechanical difference. "Bulb" is the traditional Akari term. "Cat" is GridPaw's themed version. Both illuminate their row and column identically.
No. Two cats cannot be in the same unobstructed row or column. If they can see each other, that is a rule violation that must be fixed.
A cat's light travels in all four directions (up, down, left, right) until it hits a black wall or the edge of the grid. There is no distance limit.
Light Up / Illuminate — To "light up" or "illuminate" a cell means that at least one bulb's line of sight reaches it. When you place a bulb, it illuminates every white cell in its row and column until blocked by a wall.
Each bulb casts light in four cardinal directions. The light passes through white cells but stops at black walls. A white cell is "lit" if any bulb's light reaches it from any direction. The win condition requires all white cells to be simultaneously illuminated.
Nothing bad — it is perfectly valid for a cell to be illuminated by multiple bulbs. It is just inefficient. The puzzle only requires that every white cell is lit at least once.
When every white cell is illuminated, all numbered cell constraints are satisfied, and no two bulbs can see each other. GridPaw shows a celebration screen when you win.
X Mark — An X mark is a player-placed annotation on a white cell to indicate that no bulb should go there. X marks are not part of the official Akari rules — they are a solving aid.
Most online Akari implementations, including GridPaw, let you place X marks by right-clicking or long-pressing a cell. X marks help you track elimination logic and avoid accidentally placing bulbs in impossible positions.
No. X marks are a convenience tool, not a rule. Experienced solvers often skip them on easy puzzles. But on hard puzzles, they are invaluable for tracking complex deductions.
Right-click on desktop or long-press on mobile to toggle X marks on any white cell. Click again to remove the mark.
Black Wall — A black wall is any dark cell in the grid that blocks light propagation. Black walls come in two types: plain walls (no number) and numbered walls.
Plain walls simply block light — they carry no adjacency constraints. Numbered walls enforce a specific bulb count on their neighbors. Both types stop light from passing through, which is the core spatial mechanic of Akari.
No. Black walls completely block light. A bulb's illumination stops the moment it hits a wall in any direction.
A plain wall just blocks light. A numbered wall blocks light AND requires its adjacent white cells to contain a specific number of bulbs.
Elimination — Elimination is the process of ruling out impossible bulb placements based on logical deduction. When you determine that a cell cannot contain a bulb, you eliminate it.
Elimination works by contradiction: if placing a bulb in a cell would violate a numbered constraint, create a bulb conflict, or leave another cell permanently dark, then that cell cannot hold a bulb. Every step forward in Akari comes from either placing a bulb or eliminating a position.
Elimination means ruling out cells where a bulb cannot go. You determine this by checking if a placement would violate any numbered constraint or create a bulb conflict.
No. Elimination is pure logical deduction — you prove a cell cannot hold a bulb based on existing constraints. Guessing means placing a bulb without proof and seeing if it works.
Constraint Propagation — Constraint propagation is the foundational solving technique in every Akari puzzle. It works by examining numbered cells and deducing forced placements or eliminations.
Start by examining numbered cells. A "4" forces all four neighbors to be bulbs. A "0" eliminates all four neighbors. After each placement, propagate the consequences — newly placed bulbs illuminate rows and columns, which may trigger further deductions. Repeat until no more simple deductions are available.
It is the process of examining numbered cells, deducing forced placements or eliminations, and then propagating those consequences to find more deductions. It is the primary solving technique for Easy and Medium puzzles.
Easy and Medium puzzles, usually yes. Hard puzzles on larger grids may require additional techniques like elimination chains or backtracking.
Backtracking — Backtracking is an advanced solving technique — and the method computer solvers use when simple deduction is not enough. You pick an undecided cell, assume it holds a bulb, and propagate all consequences.
Choose an undecided cell and mentally assume "a bulb goes here." Propagate all consequences — illumination, neighbor counts, conflict zones. If you reach a contradiction (a numbered cell gets too many bulbs, or a white cell becomes impossible to illuminate), your assumption was wrong and that cell cannot hold a bulb. If no contradiction appears, continue until the puzzle is solved.
No. Guessing means placing a bulb without logical justification. Backtracking is a systematic proof technique: you assume a position, derive all consequences, and check for contradictions. It is logically rigorous.
Only when constraint propagation and elimination are exhausted. On Easy and Medium puzzles, you should never need backtracking. On Hard 12×12+ grids, it may be necessary.
Yes. GridPaw's built-in solver uses constraint propagation as its first pass, then falls back to backtracking when simple deduction is insufficient.
Akari — Akari (あかり) is the original Japanese name for this logic puzzle, meaning "light." It was created and published by Nikoli in 2001.
Akari is played on a rectangular grid of white and black cells. Some black cells carry numbers from 0 to 4. Players place light bulbs on white cells so that every white cell is illuminated, numbered constraints are satisfied, and no two bulbs see each other in the same row or column.
Akari (あかり, "light" in Japanese) is a logic puzzle by Nikoli where you place light bulbs on a grid to illuminate every white cell while following numbered clues. GridPaw is the cat-themed version.
Yes. "Akari" is the original Japanese name by Nikoli. "Light Up" is the English localization. Same puzzle, same rules.
Nikoli, the Japanese puzzle publisher behind Sudoku, Kakuro, and dozens of other logic puzzles. Akari was first published in 2001.
Light Up Puzzle — A "light up puzzle" is the English name for Akari. The two terms are completely interchangeable — same rules, same logic, same solving techniques.
Light Up became the standard English localization when Nikoli's puzzles spread internationally through newspapers and online puzzle sites. Some players prefer "Light Up" because it is self-descriptive, while purists and Japanese puzzle fans stick with "Akari."
Yes. "Light Up" is the English name for Akari (あかり). They are identical puzzles with the same rules.
Either is fine. "Akari" is preferred by puzzle purists and Japanese puzzle fans. "Light Up" is more common in English-language contexts.
Nikoli — Nikoli (ニコリ) is the legendary Japanese puzzle publisher that created Akari, Sudoku, Kakuro, Slitherlink, and dozens of other logic puzzles. Founded in 1980.
Nikoli's editorial philosophy is strict: every puzzle must have a unique solution solvable through pure logic alone — no guessing, no trivia, no luck. This philosophy makes Nikoli puzzles the gold standard for logic puzzle quality worldwide.
Nikoli is a Japanese puzzle publisher founded in 1980. They created Akari, Sudoku, Kakuro, Slitherlink, and dozens of other logic puzzles. Their editorial standard requires every puzzle to have a unique solution.
Nikoli popularized Sudoku and gave it its modern name, but the concept of number-placement grids existed earlier. Nikoli refined the puzzle and published it in their magazine starting in 1984.
Daily Challenge — A daily challenge is a new Akari puzzle released every day for players to solve. Daily challenges create a solving habit and give players a shared experience.
On GridPaw, the daily challenge offers a fresh 14×14 puzzle each day with a balanced difficulty level. The same puzzle is shown to all players worldwide, creating a shared daily ritual. Streak tracking rewards consecutive days of solving.
A new 14×14 Akari puzzle released every day. Everyone worldwide gets the same puzzle. Solve it to build your streak and earn badges.
A new puzzle appears at midnight UTC every day. Your streak continues as long as you solve each day's puzzle before the next one arrives.
Understanding akari terms is essential for improving your solving speed and accuracy. When you know what constraint propagation means, you can apply it deliberately instead of stumbling into deductions by accident. When you understand backtracking, you know exactly when to stop relying on simple logic and start testing hypotheses. This puzzle glossary is designed to be your quick-reference companion — keep it open in a tab while you play and look up any term you encounter.
The akari glossary on this page covers every term from beginner concepts like number cells and bulbs to advanced techniques like constraint propagation and backtracking. As the GridPaw site grows, we will continue expanding this akari glossary with new terms, tips, and cross-references to help you become a better solver.
There is no difference. "Akari" is the original Japanese name by Nikoli, and "Light Up" is the English localization. Both terms refer to the same logic puzzle with identical rules. This akari glossary uses both names interchangeably.
No. The essential terms for beginners are number cell, bulb (or cat), black wall, and illumination. The solving technique terms like constraint propagation and backtracking become useful once you move beyond Easy puzzles. Start playing and refer back to this puzzle glossary as needed.
Play the daily Akari challenge on GridPaw for a fresh puzzle every day, or try our how to play guide for a step-by-step introduction. You can also explore the Akari solver to see constraint propagation and backtracking in action.
