Solving an 8×8 sudoku:
what 2×4 blocks change

The grid holds 64 cells and is filled with the digits 1 to 8. The rules do not change: every row, every column and every block contains each digit exactly once.

What changes is the shape of the blocks. Each is two rows tall and four columns wide, which gives eight blocks arranged in four rows of two:

The eight blocks of an 8×8 sudoku: two rows, four columns each.

Two consequences follow from that layout, and they are the whole article:

• a row crosses only 2 blocks, instead of 3 in classic sudoku;
• a column crosses 4.

The 9×9 is symmetrical — rows and columns play exactly the same role. The 8×8 is not, and that imbalance is what you exploit.

Scanning is the basic technique: to place a digit inside a block, you rule out every cell whose row or column already holds that digit elsewhere. In a 9×9, ruling out a row or a column amounts to the same thing — 3 cells eliminated out of 9 either way.

In an 8×8, not at all. A block is two rows of four cells:

• ruling out a row eliminates 4 cells out of 8 — half the block at once;
• ruling out a column eliminates only 2.

This also gives you a free error check. Since every block must contain the digit exactly once, that digit cannot sit outside the block on both of its rows at the same time. If you think you are seeing that, the puzzle is not broken: a digit was misplaced earlier.

Call a band a pair of side-by-side blocks — that is, two full rows of the grid. Each of those two rows holds the digits 1 to 8 once, so the band holds every digit exactly twice. And since each block holds it once, there is necessarily one in the left block and one in the right block.

Those two copies cannot share a row — that row would hold the digit twice. Hence the deduction, immediate and free:

The 5 in the right block has to be on the bottom row. Four cells left to separate.

In classic sudoku, the same band has three blocks and three rows: placing a digit in one of them still leaves two possible rows for each of the other two. Here there is no choice at all — the constraint is total. It is the single most profitable reflex in an 8×8: every time you place a digit, look straight at the neighboring block in the same band.

Top to bottom, the grid reads as two stacks of four blocks. A stack is 4 columns by 8 rows, so it holds every digit four times — once per block. But each of its four columns can hold it only once, so those four copies occupy the four columns, one each.

That is a strong constraint and an easy one to read. As soon as the digit is placed in three of a stack's four blocks, its column in the fourth is settled: it is the only one left free. Only two cells remain possible — the block's two rows — and the two-row rule often settles the second.

Combining the two is what cracks most Easy and Medium puzzles: the band gives you the row, the stack gives you the column, and the intersection gives you the cell. Without writing a single pencil mark.

Eight candidates per cell is already too much to hold in your head past the Easy level. On Sudoku-Land, notes mode is one right-click away on any empty cell, or one tap on the dedicated button of the input ring.

Once the marks are down, the technique that pays best in an 8×8 is the naked pair: two cells in the same block holding the same two candidates and nothing else. Those two digits are reserved for them, and vanish from the block's six other cells.

And it pays more than in a 9×9, for a purely geometric reason. In a 2-by-4 block, two cells picked at random share a row 3 times out of 7 — against 2 out of 8 in a square block. A pair that lines up does not only eliminate inside its block: it eliminates along the whole row too. So nearly one naked pair in two hands you that double effect.

The rest of the classic arsenal works unchanged: hidden singles, hidden pairs, and even the X-Wing on Hard puzzles — reading columns rather than rows, since they take longer to constrain.

1. Looking for squares. An eye trained on the 9×9 carves the grid into square blocks by itself, and gets it wrong every other cell. Trust the thick lines, not your intuition — especially in the first few minutes.
2. Scanning by columns. It is the mirror reflex from classic sudoku, and here it yields half as much: two cells eliminated instead of four. Rows first, always.
3. Forgetting the neighboring block. The two-row rule is the easiest gain on the board, and it is the one you skip when playing fast. Digit placed, glance next door.

Junior+ Sudoku 8×8 offers free puzzles at three levels — Easy, Medium and Hard — with notes mode, automatic saving of the game in progress, and the 🆘 Help button, which animates the grid to show what to do next when you get stuck.

If the 8×8 still fights back, Junior Sudoku 6×6 trains the same reflexes on 2×3 blocks with only six candidates per cell. And once the Hard puzzles fall, classic 9×9 sudoku has little left to teach you — other than being symmetrical again.