Sudoku X for beginners

This article assumes you've played classic Sudoku before, even casually, and you're now sitting in front of your first Sudoku X — also called Diagonal Sudoku — wondering what you're meant to do differently. The short answer: not much. The slightly longer answer is below.

Before you start

Open an easy Sudoku X puzzle in another tab. Look at the board. Notice the seventeen cells marked with a dotted ring — those are the cells on the two diagonals. One diagonal runs from the top-left corner to the bottom-right corner. The other runs from the top-right corner to the bottom-left corner. The two cross at the centre cell, which sits on both.

Those seventeen cells participate in one extra rule compared to classic Sudoku: each of the two diagonals has to contain every digit from 1 to 9 exactly once. The other 64 cells follow the standard row, column, and box rules with no addition.

That's everything you need to know to start.

The first move

Pick a digit — say, 1 — and scan the board for it the way you would in classic Sudoku. Look in each row, column, and 3×3 box for cells where 1 is the only candidate left. Place the easy ones first.

When you place a 1 on a cell that sits on a diagonal, the diagonal now contains a 1, which eliminates 1 from the other eight cells on that diagonal. This is the only new thing to track. Everything else is identical to classic Sudoku.

When the diagonal becomes useful

Most of an easy Sudoku X solves by classic-style scanning alone. The diagonals just give you more places where naked and hidden singles fall into your lap. Don't overthink the diagonals on your first puzzle; play it like a classic and watch where the diagonal cells happen to land.

By the middle of the puzzle, you may hit a cell where row, column, and box reasoning haven't narrowed the candidates down enough. Before reaching for a more complex technique, check: does this cell sit on a diagonal? If yes, what digits are already on that diagonal? Eliminate them from your candidate list. About a third of the time, that's the missing constraint.

A common beginner mistake

The most common stumble for solvers on their first Sudoku X is forgetting the diagonals exist. The mistake usually goes like this: you scan a row, scan a column, scan a box, find no candidates that are forced, and assume the puzzle requires a harder technique. Then you spend three minutes considering pairs and chains, finally place the digit somewhere, and look up to realise the diagonal already contained that digit two rows up — it had been impossible all along.

The fix is mechanical: when you're working on a cell that's marked with a dotted ring, the diagonal is one of the units you scan, the same as row, column, and box. After your third or fourth puzzle, this becomes automatic.

What "done" looks like

A solved easy Sudoku X has every row, column, box, and both diagonals containing each digit 1 through 9 exactly once. The completion overlay on Sudoku Mountain triggers when the grid is logically complete; you won't accidentally submit a wrong puzzle.

If you finish your first one in fifteen minutes, that's normal. If it takes twenty-five, also normal — the diagonal scan is a new habit, and habits are slow before they're fast.

When you're ready for medium

After three or four easy Sudoku X puzzles, you'll notice the diagonal scan has become automatic. Your eye starts to look at the diagonals without your conscious effort. That's the moment to try medium. The mediums have fewer givens to start with and lean more heavily on the diagonal — they're where the variant actually starts to feel like its own puzzle rather than a slightly-enriched classic.

If you'd like a deeper look at the moves that become available once the diagonal is in play, when the diagonal helps walks through the three most common patterns. But honestly, you can also just play the next puzzle. The diagonal teaches itself.