Each one is shown on a real grid, at the exact moment it applies.
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Learn one of these once and you carry it to every sudoku you ever meet. Each is shown on a classic grid, because each works on a classic grid.
A cell has only one possibility left, because every other digit already appears in its row, its column or its region.
Within one row, column or region there is only one cell that can take a digit — even if that cell still has other candidates of its own.
If a digit can only go in one row (or column) inside a region, it is ruled out of the rest of that row outside the region.
Two cells in the same unit hold the same two candidates, so those two digits belong to those two cells and are ruled out everywhere else in the unit.
Two digits in a unit can only go in the same two cells, so every other candidate in those two cells is ruled out.
Three cells in a unit share just three candidates between them, so those three digits are ruled out of every other cell in the unit.
A digit fits in only two cells in each of two rows, and those cells line up in the same two columns — so it is ruled out of the rest of both columns.
Three two-candidate cells (xy, xz, yz) link up, so any cell that sees both ends cannot hold z.
Follow a chain of one digit in two alternating colours. If two cells of the same colour meet inside one unit, that whole colour is wrong.
Assume a value, follow it until something contradicts, and conclude the assumption was wrong — the last resort when every other technique has stalled.
What each variant adds to classic sudoku. These are not skills you can take elsewhere — they are the rule that makes that game itself, and they mean nothing on a board that does not have it.
Fill in the digits 1 to 9 so that no digit repeats in any row, any column, or any 3×3 box.
Fill in the letters A to I so that no letter repeats in any row, any column, or any 3×3 box.
Fill in the Thai letters ก, ข, ค, ง, จ, ฉ, ช, ซ, ฌ so that no letter repeats in any row, any column, or any 3×3 box.
Fill in the digits 1 to 9 so that no digit repeats in any row, any column, any 3×3 box, or either main diagonal.
Fill in the digits 1 to 9 so that no digit repeats in any row, any column, or any region outlined in bold.
Fill in the digits 1 to 9 so that no digit repeats in any row, any column, or any 3×3 box. Every shaded cell must contain an even number.
Fill in the digits 1 to 9 so that no digit repeats in any row, any column, or any 3×3 box. A black bar between two cells means those two numbers are consecutive; where there is no bar between two neighbouring cells, their numbers must NOT be consecutive.
Fill in the digits 1 to 9 so that no digit repeats in any row, any column, or any 3×3 box, and the 9 shaded cells also contain 1 to 9.
Fill in the digits 1 to 9 so that no digit repeats in any row, any column, or any 3×3 box, and each of the four shaded 3×3 windows also contains 1 to 9.
Fill in the digits 1 to 9 so that no digit repeats in any row, any column, any region outlined in bold, or either main diagonal.
Nothing is filled in to start — the cages are the puzzle. Fill in the digits 1 to 9 so that no digit repeats in any row, any column, or any 3×3 box. The digits inside each dashed cage must add up to the small number in its corner, and no digit may repeat inside a cage.
Fill in the digits 1 to 9 so that none repeats in any row, column or 3x3 box — and no two cells a chess knight's move apart hold the same digit.
Fill in the digits 1 to 9 so that none repeats in any row, column or 3x3 box. Every border inside a 3x3 box carries a < or > sign: the point of the sign faces the smaller of the two numbers.
Fill in the digits 1 to 9 so that none repeats in any row, column or 3x3 box. Along every grey thermometer the numbers must increase, starting from the round bulb.
Fill in the digits 1 to 9 so that none repeats in any row, column or 3x3 box. The number in each circle is the sum of the numbers along the arrow leaving it; digits may repeat along an arrow.
Fill in the digits 1 to 6 so that no digit repeats in any row or column. There are no blocks. Each dashed cage shows a number and a sign: the digits inside it must make that number using that operation. A digit MAY repeat inside a cage, as long as the row and column rule holds.
Put one of each picture in every row, every column and every box. No numbers and no reading — just look for the picture that is missing.
Every example is a real generated puzzle, checked to contain this technique.