The lesson files
Follow the logic.
Solving techniques, from singles to chains. Watch a worked example, then try the idea in a puzzle.
Looking for what a rule means? See Variants
Singles
Pairs, triples and quads
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Classic Sudoku: Use a pair of cells to remove candidates
A naked pair is two cells that can hold the same two digits and no others.
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Classic Sudoku: Find two digits confined to two cells
A hidden pair is two digits that have only the same two places in a row, column or box.
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Classic Sudoku: Reserve three digits across three cells
A naked triple works like a pair, with three cells reserving three digits.
Reading the rules
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Thermo Sudoku: Work inwards from both ends of a thermometer
The bulb is three, and the tip is six.
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Killer Sudoku cage combinations: find the digits in a small cage
A three-cell Killer cage totalling 6 must contain 1, 2 and 3.
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Kropki Sudoku: Find the partner across a dot or marker
A dot or letter gives the two cells a relationship.
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Renban Sudoku: Discover which rule a line follows
This line follows one of two rules.
Sums and totals
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Killer Sudoku rule of 45: find a missing digit in a row
Every Sudoku row totals 45.
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Killer Sudoku: Find an outie from a Killer box
A Sudoku box also contains one through nine, so it totals forty-five.
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Arrow Sudoku: Use the rule of 45 to find an arrow circle
Here, two Killer cages and an arrow meet in one Sudoku row.
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Killer Sudoku: Use a sum you worked out as an invisible cage
These two Killer cages cover six cells of the top row.
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Parity line Sudoku: Count the odds to orient a parity line
A parity line alternates odd and even digits.
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Region sum Sudoku: Use a complete unit to find equal sums
This region sum line covers an entire row.
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Killer Sudoku: add cage sums to find a shared digit set
Combine two Killer cage totals to identify four digits, split them between the cages, then use a column clue to finish the deduction.
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Arrow Sudoku: Subtract an overlapping cage from an arrow
This arrow and this Killer cage overlap.
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Killer Sudoku: Find two missing cells with the rule of 45
A row always totals forty-five.
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Killer Sudoku: Use a box to constrain a crossing Killer cage
This fourteen-cage crosses a box edge.
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Classic Sudoku: Count matching digits in the corners and centre ring
There is a surprising match on a nine-by-nine Sudoku.
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Killer Sudoku: Compare two Killer cages through their shared row
These two cages do not overlap.
Colouring
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German whisper Sudoku: Join two Whisper colour patterns where they meet
These are two separate German Whisper lines, meeting at one cell.
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Parity line Sudoku: Use an alternating line to claim every even digit
An alternating line can affect cells beyond the line.
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Killer Sudoku: Use a modular line to choose a Killer cage pair
This fifteen-cage sits on the start of a modular line.
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German whisper Sudoku: Count low and high digits around a Whisper line
A German Whisper jumps at least five between neighbours.
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Modular Sudoku: Rule out an impossible modular group
A modular line repeats three digit groups: one-four-seven, two-five-eight and three-six-nine.
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Kropki Sudoku: Count even digits through V and white dots
Two V marks and two white dots make four separate pairs in this row.
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Renban Sudoku: Count odd and even digits beside a Renban line
A Renban line holds consecutive digits in any order.
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Entropic Sudoku: Use a row quota to colour an entropic line
This entropic line has four cells.
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Isotropic Sudoku: Count a same-class line against its row
All three cells on this isotropic line use one digit band.
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Modular Sudoku: Count a modular class across a long line
On a modular line, each run of three cells uses one digit from each group.
Dot chains
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Kropki Sudoku: Follow a chain of black dots
Three black dots join these four cells.
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Kropki Sudoku: Read two dots together
One black dot asks for a double or a half.
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Kropki Sudoku: Carry a Whisper’s side through a black dot
These two green Whisper lines are linked by one black dot.
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Kropki Sudoku: Let a Whisper choose between dot-chain options
Two black dots meet here.
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XV (negative) Sudoku: Let a missing V settle an X chain
Every X and V in this puzzle is shown.
Following a rule through
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Arrow Sudoku: Reject an arrow bulb that empties its box
This arrow sits almost entirely inside one Sudoku box.
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German whisper Sudoku: Test a candidate through meeting Whisper lines
Two German Whisper lines meet here.
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Killer Sudoku: Let a Killer cage rule out digits nearby
This small Killer cage sums to three.
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Kropki Sudoku: Use a black dot to protect a box’s last one
This black dot sits inside the top-left box.
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Modular Sudoku: Let a modular line protect the box’s last one
This modular line crosses the top of a Sudoku box.
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Region sum Sudoku: Reject a line value that empties the next box
This region sum line crosses a box border.
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Renban Sudoku: Reject a Renban digit that leaves its box empty
This Renban line sits along the bottom of a Sudoku box.
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Thermo Sudoku: Reject a thermometer start that empties its box
This thermometer stays inside the centre box.
