SudokuBoku

Technique trainer One move at a time

Choose a move. You get a real position where it is the only one left, pencil marks included.

00:00 Mistakes 0
Rules
  • Fill every row, every column and every 3×3 box with the digits 1 to 9.
  • A digit can appear once in each of those — never twice.
  • There is exactly one solution, and logic alone will reach it.
  • Rows and columns each take the digits 1 to 9, once apiece.
  • The boxes are irregular shapes, drawn with the heavy outlines.
  • However a region bends, no digit repeats inside it.
  • The colour patches are cages; the small number is what a cage's digits add up to.
  • No digit repeats within a cage — nor in any row, column or 3×3 box.
  • There are no given digits: the sums alone pin down the whole grid.
  • Everything from classic applies: rows, columns and 3×3 boxes take 1 to 9.
  • The two tinted diagonals must each hold 1 to 9 exactly once as well.
  • All the classic constraints hold — rows, columns and 3×3 boxes take 1 to 9.
  • Each of the four shaded windows must also contain 1 to 9 exactly once.
  • The windows sit a step in from the corners, overlapping four boxes apiece.
  • Rows, columns and 3×3 boxes work exactly as they always have.
  • On top of that, two squares a chess knight's move apart may not hold the same digit.
  • Select a square and its knight partners are ringed with a dashed outline.
  • Every row, column and 3×3 box holds 1 to 9, exactly as in classic.
  • Along each grey path, digits strictly climb from the round bulb to the far end.
  • They don't have to be consecutive — each square just beats the one before it.
  • Rows, columns and 3×3 boxes take 1 to 9, exactly as in classic.
  • The digits along each arrow add up to the digit in the circle it comes from.
  • Digits may repeat along an arrow — as long as no row, column or box objects.
  • Fill every row, column and 3×3 box with the digits 1 to 9, as usual.
  • Between squares in the same box you'll find a sign; its point aims at the smaller digit.
  • Every comparison inside every box is shown — a hundred and eight of them.
  • The higher levels hand you fewer digits, not fewer signs. Expert gives you four.
  • Fill every row, column and 3×3 box with the digits 1 to 9, as usual.
  • A white dot between two squares means their digits differ by one.
  • A black dot means one digit is exactly double the other. 1 and 2 are both, and always show black.
  • A boundary with no dot is a clue too — those two digits are neither one apart nor double and half. Every boundary is marked or deliberately bare.
  • Expert hands you no digits at all. The dots and the gaps are the whole puzzle.
  • Fill every row, column and 3×3 box with the digits 1 to 9, as usual.
  • A square with a ring behind it holds an odd digit — 1, 3, 5, 7 or 9.
  • A square with a frame behind it holds an even one — 2, 4, 6 or 8.
  • Most squares are marked with neither, and those are genuinely unrestricted.
  • Every square switches to the fully marked board — one level, gentler than it looks, and two puzzles sharing a grid rather than one.
  • Every row, column and box holds five odd digits and four even ones — so once five squares of a house are known odd, the rest of it is even, marked or not.
  • Solve the 9×9 sudoku as usual — every row, column and box takes 1 to 9.
  • Ten ships are hidden on the same squares: one of four, two of three, three of two and four single squares.
  • Ships are straight, and no two of them touch — not even at a corner.
  • The numbers beside and below say how many ship squares each row and column holds.
  • A ship listed with digits sits on exactly those digits, in order — so the grid is your map.
  • Five 9×9 grids interlock: four corners and a centre, each corner sharing one 3×3 box with it.
  • Every grid obeys ordinary sudoku on its own — rows, columns and boxes each take 1 to 9 once.
  • A tinted box belongs to two grids at once, and only together do the five have one solution.
  • Fill every row, every column and every box with the digits 1 to 6.
  • The boxes are three squares wide and two tall.
  • Nothing repeats in any row, column or box.
  • Put 1, 2, 3 and 4 into each row, each column and each 2×2 box.
  • No digit may appear twice in any of them.
  • Every square can be worked out — no guessing needed.
Keyboard shortcuts
1–9
Place a digit, or toggle a pencil mark
Arrows
Move around the grid
N
Notes mode on or off
Backspace
Clear the square
Ctrl + Z
Undo · Ctrl + Y redo

Why the position, not the puzzle

The usual way to practise a technique is to find a puzzle said to contain one and solve from the top. That means forty moves of scanning before you reach the interesting part, and by the time you get there you have forgotten what you came for. Worse, you often cannot tell whether the move you finally made was the one the puzzle was chosen for.

This page hands you the position instead. Every board here has been played forward by the same solver that grades our puzzles, stopping at the exact step where the technique you asked for becomes the move to make. The digits already on the grid are fixed — they are the setup, not your work — and the pencil marks are the ones the solve had at that moment.

Nothing easier applies. If a naked single were sitting on the board you would find it instead, and learn nothing — so a position is only offered when every cheaper technique has already run out.

The marks are part of the position

You are given pencil marks, which is unusual for a puzzle site and is not a convenience. On the upper half of the ladder there is generally no position at all where the technique is the move if you only have digits to look at: by the time an X-wing is what you need, a solver has been writing candidates for twenty moves, and the pattern lives in those notes rather than in the grid. Hand over the digits alone and the position quietly stops being the exercise — some cheaper move reappears, and you take it.

So the marks travel with the board, exactly as the solver had them. What you are looking at is a faithful snapshot of somebody's grid partway through a hard puzzle, which is the only honest way to practise the moves that only happen partway through hard puzzles.

What is offered, and what is not

The two singles are missing on purpose: they are the moves nobody needs drilling, and a position whose cheapest move is a single is just an ordinary puzzle. Rungs belonging to another board are missing too — the Law of Leftovers cannot occur on a square grid, and the diagonal and knight moves need their own variant's rules.

One absence is worth explaining, because it is a measurement rather than a decision. There is no practice for the jellyfish. Across sixty-seven thousand positions taken from more than a thousand puzzles at five different clue counts, a jellyfish was the cheapest move available exactly once. It is not that the pattern is hard to detect — it is that anything a four-line fish can do, a smaller fish or a wing has almost always done already. A technique you will never be obliged to use is not a technique worth drilling, and pretending otherwise would waste your time twice over.

The swordfish sits just inside that line and takes a moment to find, which is why the board sometimes thinks for a second or two before it appears. Everything else arrives more or less instantly. If a search comes back empty, asking again is the right response — the scarce moves are genuinely scarce, and the page would rather say so than serve you a position that does not keep its promise.

Using it well

Take the hint button seriously here: it names the move and shows you where, which on this page is the answer rather than a nudge. Try to find the pattern first, then use the hint to check whether what you found is what the solver found — those come apart more often than you would expect, and the disagreement is where the learning is.

When a technique starts feeling obvious, move down the list rather than up. The ladder here runs roughly in the order a solver meets these moves, and the reason people stall is almost never the exotic pattern at the bottom — it is a locked candidate or a hidden pair two rungs above it that they never quite made automatic.