Where did sudoku originate?

The modern puzzle originated in the United States, published as "Number Place" in a Dell puzzle magazine in May 1979. It was picked up in Japan by the publisher Nikoli in 1984, renamed sudoku, and returned to the wider world through British newspapers in 2004.

The name is Japanese and Japan is where the puzzle was refined and popularized, but the game itself was first published in America. Calling it a Japanese puzzle is about half right — the naming and the editorial craft are Japanese, the grid is not.

Howard Garns, a retired Indianapolis architect, is widely credited with creating the 1979 Number Place puzzle, based on his name appearing in exactly those Dell issues that contained it. The attribution has never been officially confirmed, and Garns died in 1989 without seeing the puzzle become famous.

Did Euler invent sudoku?

No. Euler studied and named Latin squares in the 1770s and 1780s, which share sudoku's row and column rules but have no 3×3 box constraint. That box constraint is what makes sudoku a solvable puzzle rather than a combinatorial object, and it isn't in his work.

When did sudoku become popular?

Late 2004 and through 2005. Wayne Gould's generated puzzles reached The Times of London in November 2004, other papers followed within weeks, and the craze spread worldwide during 2005. The first World Sudoku Championship took place in Italy in 2006.

Why is it called sudoku?

It's a contraction of the Japanese phrase Nikoli originally published it under, sūji wa dokushin ni kagiru — roughly "the digits must occur only once." Nikoli's Maki Kaji shortened it to sudoku, from su (number) and doku (single).

What Is Candidate Mode in Sudoku? Digital Pencil Marks Explained

Candidate mode is a feature in most digital sudoku apps that lets you write small "notes" (possible numbers) into a cell instead of committing to a single final answer — it's the digital equivalent of pencil marks on paper, and it's essential for anything beyond an easy puzzle. Switching between candidate mode and regular entry mode is one of the first things worth understanding when using any sudoku app.

Candidate Mode vs. Regular Entry Mode

Regular (normal) entry mode: tapping a number places it as your final answer for that cell, displayed large and centered, exactly as if you'd written it in pen.

Candidate mode: tapping a number adds it as a small note in the corner of the cell instead of a final answer — you can add multiple candidate numbers to the same cell, representing every number you think could still legally go there. These small notes don't count as your final answer; they're just a tracking tool while you work out what's actually correct.

Most apps let you toggle between the two modes with a single button, often labeled "notes," "pencil marks," or "candidates."

Why Candidate Mode Matters So Much

Beyond the easiest difficulty level, sudoku becomes genuinely difficult to solve purely in your head — there's simply too much information to track reliably across 81 cells. Candidate mode solves this by externalizing that tracking: instead of trying to remember "cell (4,7) could be a 2, 5, or 8" in your head, you write it directly in the cell and can refer back to it instantly.

This isn't a beginner crutch — virtually every experienced sudoku solver uses candidate mode (or paper pencil marks) on anything beyond an easy puzzle, since many of the core solving techniques (naked pairs, pointing pairs, and especially advanced techniques like X-Wing) depend entirely on comparing candidate lists across multiple cells — they're essentially impossible to spot without accurate, complete candidate marks in place.

How to Use Candidate Mode Effectively

Fill in candidates completely, not selectively. Once you switch to candidate mode for a cell (or use an "auto-fill candidates" feature if your app has one), write in every number that's legally possible in that cell, not just the ones that seem most likely — partial candidate lists can hide the exact patterns you need to spot.

Use an auto-update feature if your app has one. Many sudoku apps automatically remove a candidate from other cells in the same row, column, and box the moment you place a final number elsewhere — this saves significant manual work and reduces a very common source of errors (forgetting to manually remove an outdated candidate).

Switch back to regular mode once you're confident. Once you've identified a cell's final answer (through a naked single, hidden single, or any other technique), switch out of candidate mode to place it as a real answer, which also usually triggers automatic candidate cleanup in cells that share that row, column, or box.

Common Candidate Mode Mistakes

Forgetting to remove outdated candidates after placing a number. If your app doesn't auto-update candidates, this is the single most common source of solving errors — a candidate number left in a cell after it's been legally ruled out can lead you to a false naked or hidden single.

Only pencil-marking some cells. Skipping candidate marks on cells that seem "obviously easy" can mean missing a pattern that technique-based solving (like naked pairs) would have caught if every relevant cell had complete candidates.

Confusing candidate mode with regular mode mid-puzzle. Placing a number in the wrong mode by accident — entering a real answer as a candidate note, or vice versa — is an easy mistake when switching back and forth quickly; most apps show these visually differently (candidates small and in a corner grid, final answers large and centered) specifically to help prevent this confusion.

Auto-Fill Candidates: A Related but Different Feature

Many sudoku apps offer a separate but related feature often called "auto-fill candidates" or "show all candidates," which automatically calculates and displays every legally possible number for every empty cell in one action, rather than requiring you to manually tap each candidate into each cell one at a time. This is a genuine time-saver, especially on harder puzzles where a full manual pencil-marking pass can take several minutes. It's worth understanding this is a distinct feature from candidate mode itself — candidate mode is the mode that lets you add small notes instead of final answers, while auto-fill is a shortcut that populates those notes automatically rather than requiring manual entry for every cell.

Some solvers deliberately avoid auto-fill even when it's available, preferring to manually work out and enter candidates themselves as part of the solving process, since the act of manually determining candidates reinforces the underlying scanning skill in a way that instantly generated candidates don't. Others view it purely as a time-saver with no downside, since the actual logical deduction work happens after the candidates are populated either way. Neither approach is wrong — it comes down to whether you're using the puzzle purely for entertainment or specifically to build and practice your own scanning skills.

When to Turn Candidate Mode Off Again

Once a puzzle is largely solved and only a handful of cells remain, some solvers find it faster to switch back to regular entry mode entirely and finish the remaining cells directly, rather than continuing to toggle between adding candidates and confirming final answers. There's no fixed rule for exactly when to make this switch — it's a matter of personal workflow — but being aware that you can move fluidly between the two modes throughout a single puzzle, rather than picking one mode and sticking with it for the entire solve, often makes for a smoother, faster overall solving experience.

How Candidate Mode Changed Sudoku Accessibility

It's worth appreciating how significantly candidate mode (and its digital automation) has changed who can realistically enjoy harder sudoku puzzles. Before digital tools were widely available, tracking candidates for a genuinely hard puzzle meant careful, cramped handwriting in tiny cell corners, with any correction requiring careful erasing that risked smudging or losing other marks nearby. Digital candidate mode with auto-update removes this friction entirely, which has made hard, expert, and extreme puzzles considerably more approachable for casual solvers who might have found the manual bookkeeping of paper pencil marks too tedious to bother with. This is a meaningful part of why sudoku apps have broadened who regularly attempts and enjoys harder difficulty tiers compared to the print-only era of the puzzle's earlier popularity.

Choosing an App With Genuinely Good Candidate Mode Implementation

Not all sudoku apps implement candidate mode equally well, and it's worth trying a few before settling on one for regular use if you solve digitally often. Look specifically for auto-update (candidates removed automatically when a number is placed elsewhere in the same row, column, or box), an easy, low-friction way to toggle between candidate and regular entry mode, and clear visual distinction between candidate notes and confirmed answers so you never lose track of which is which mid-solve. A well-implemented candidate mode genuinely speeds up solving on harder puzzles, while a clunky implementation can slow you down enough that some solvers find themselves reverting to paper and pencil marks instead, even when a digital option is available.

Candidate Mode as a Teaching Tool for New Solvers

Beyond personal use, candidate mode has real value as a teaching aid when introducing sudoku to a new solver alongside you. Watching candidates populate and update in real time as numbers are placed makes the underlying elimination logic visually concrete in a way that's harder to convey through verbal explanation alone — a new solver can directly see why a candidate disappears from a cell the moment a matching number is placed elsewhere in that row, column, or box, which often accelerates genuine understanding of the core rule far faster than reading rules on paper.

Entering Candidates Quickly on a Phone

On a desktop the toggle is trivial — a key or a click and you are in note mode. On a phone, where most casual solving now happens, the entry method makes a real difference to whether you bother marking at all, and implementations vary more than they should.

Long-press entry. Holding a digit on the keypad enters it as a candidate instead of a placement, with no mode switch at all. It is the fastest method for marking a handful of cells, and awkward for marking the whole grid.

A persistent toggle. A pencil icon that stays on until you turn it off. Better for a full marking pass, and the source of the single most common error in digital solving: placing a digit while still in note mode, or noting one while intending to place it. Apps that visibly change the keypad color in note mode prevent most of that.

Multi-select and swipe. The better implementations let you select several cells and apply one candidate to all of them, which turns a five-minute marking pass into about a minute.

Whichever an app offers, check one thing before committing to it: that placing a digit automatically strips that candidate from every peer cell. Doing it by hand on a phone is slow enough that most solvers stop doing it, and stale candidates on a hard grid are unrecoverable.

Frequently Asked Questions

Is candidate mode the same thing as pencil marks on a printed sudoku puzzle? Yes, functionally — candidate mode in a digital app serves the exact same purpose as writing small candidate numbers by hand in the corner of a cell on a printed puzzle. It's just a digital, often auto-updating version of the same technique.

Do I need to use candidate mode for easy sudoku puzzles? Not usually — easy puzzles are typically solvable through direct scanning and simple deduction without needing to track multiple candidates per cell. Candidate mode becomes genuinely useful starting around medium difficulty and essential for hard, expert, and extreme puzzles.

Does every sudoku app have a candidate mode feature? Most well-developed sudoku apps include some version of it, often called "notes" or "pencil marks," though the exact implementation (manual entry vs. auto-fill, auto-update vs. manual cleanup) varies between different apps and websites.

Can candidate mode make mistakes for me, or does it just track my own notes? Candidate mode itself doesn't solve anything — it's purely a tracking tool for numbers you believe are still possible in a cell. Some apps do offer an "auto-fill candidates" feature that calculates every legally possible number for you automatically, which is different from candidate mode itself but often used alongside it.

Does using candidate mode count as "cheating" in competitive sudoku? No — candidate mode and pencil marks are standard, accepted parts of solving even in competitive and tournament sudoku settings; they're a note-taking aid, not an assistance feature that solves anything for you.

Can I use candidate mode selectively on just a few cells instead of the whole grid? Yes — there's no requirement to pencil-mark every single cell; some solvers add candidates only to cells they're actively working through, though a complete pencil-marking pass is generally more effective for spotting pattern-based techniques like pairs and X-Wing.

Is there a keyboard shortcut convention for toggling candidate mode on most platforms? Conventions vary by app, but many sudoku tools use a dedicated on-screen toggle button or a number key modifier (like holding a key while tapping a number) rather than a universal standard shortcut, so it's worth checking your specific app's controls.

Can candidate mode be color-coded on any apps? Some more feature-rich sudoku apps allow color-coding individual candidates, which can help visually track a specific pattern (like a naked pair or an X-Wing chain) while solving, though this is a more advanced feature not found in every basic app.

Is there a downside to always using auto-fill candidates instead of entering them manually? The main tradeoff is reduced scanning practice — manually determining candidates reinforces the underlying skill more than instantly generated ones, so solvers specifically trying to build their own scanning speed may want to limit auto-fill use during deliberate practice.

Do accessibility features exist for candidate mode for solvers with visual impairments? Some sudoku apps include accessibility features like adjustable text size, high-contrast candidate displays, or screen-reader compatibility, though the depth of support varies significantly between different apps and platforms.

Is there a risk of over-relying on candidate mode and never developing independent scanning skill? It's a fair concern for beginners who lean heavily on auto-fill specifically, but manually entering and updating candidates yourself, even in candidate mode, still builds and reinforces genuine scanning skill over time.

What is candidate mode in sudoku? It is the digital equivalent of pencil marks: a toggle that switches number entry from placing a digit in a cell to noting a small possibility inside it. In candidate mode, tapping 4 in a cell records "4 could go here" rather than "4 goes here," and tapping it again removes the note. Every technique beyond basic scanning depends on having those notes available, which is why it is the one feature worth checking before choosing an app.