Sudoku Basics

What "the basics" of sudoku really means

Before you can do a sudoku, you need names for the things you are reasoning about. The basics are not the rules — the rules fit in a sentence (each row, each column and each 3×3 box contains 1–9 once) and they belong on the how to play page. The basics are not the solving techniques either; the deduction rules that earn names like "naked single" and "X-Wing" live in the techniques cluster. The basics are everything in between: the words you need so the rules statement reads cleanly and the technique pages make sense.

In practice that means five small bodies of knowledge. First, the parts of the grid. Second, what givens and candidates are. Third, how pencil marks are written. Fourth, the shorthand people use to point at a specific cell. Fifth, what the difficulty label on a puzzle actually means. Each one is a few paragraphs of reading; together they are the smallest set you can have and still talk about sudoku properly.

• The parts of the grid — cell, row, column, box, unit, peer.
• Givens and candidates — the fixed clues, and the digits still legal in an empty cell.
• Pencil marks — how to record candidates without cluttering the grid.
• Notation — rNcN and the way rows, columns and boxes are numbered.
• Difficulty — what easy, medium, hard, expert, master and evil actually mean.

The parts of the grid you have to know

A classic sudoku is a 9×9 grid with 81 cells. The cells sit in nine rows numbered 1 to 9 from top to bottom, nine columns numbered 1 to 9 from left to right, and nine 3×3 boxes (sometimes called regions or blocks) marked by the heavier inside lines. Rows, columns and boxes are collectively called "units" — those are the groups the rule applies to, and they are the only groups the rule applies to.

Every cell belongs to exactly three units at once: one row, one column and one box. The cells that share a unit with a particular cell are its "peers" — there are 20 of them on a 9×9 grid (the other eight in your row, the other eight in your column, and the four extra cells in your box). When you place a digit, you immediately rule it out of all 20 peers. That overlap is the engine behind every deduction, which is why the next four basics pages keep returning to it.

Two wider groupings are useful too. Three boxes side by side make a "band"; three boxes top to bottom make a "stack". You will mostly see those words on technique pages — the names matter because some patterns (like the box/line reduction) live inside a single band or stack. For now, just know they exist. The grid anatomy page walks through all of this with a labelled diagram.

Givens, candidates and pencil marks — the bookkeeping

The digits printed at the start of a puzzle are the "givens" (some sites also call them clues). They are fixed, correct, and cannot be changed. A reputable puzzle ships with exactly enough of them to force a single solution, so if a solve ever seems to require changing a given, an earlier move of your own was wrong.

A "candidate" is a digit that could still legally go in an empty cell — one that does not already appear in that cell's row, column or box. Solving is the steady process of narrowing each cell's candidates down to one. On a fresh puzzle every empty cell has up to nine candidates; as you place digits, candidates fall away. When only one candidate is left in a cell, that cell is a "naked single" and the surviving digit is forced. The givens and candidates page is the full version of this.

Pencil marks are the small candidate digits you write into an empty cell to remember which numbers can still go there. The full pencil-mark method records every legal candidate in every cell — it fills the grid with tiny numbers, but it makes patterns visually obvious. Snyder notation, named after world champion Thomas Snyder, is sparser: you go box by box and digit by digit, and you only pencil a digit when it has exactly two possible cells in that box. Many fast solvers start every puzzle in Snyder notation and add full marks only if they get stuck. The pencil marks page compares both.

• A given is a digit printed at the start; it is fixed and correct.
• A candidate is a digit still legal in an empty cell.
• Full pencil marks record every candidate; Snyder notation records only digits with two candidate cells in a box.

Notation — how to name a cell out loud

To talk about a sudoku — in a guide, a forum, your own notes, or this wiki — you need a way to name a specific cell. The standard is rNcN notation: r for the row, c for the column, each followed by its number. The cell in the third row and fifth column is r3c5; the top-left cell is r1c1; the centre cell is r5c5; the bottom-right is r9c9. The wiki uses this shorthand throughout the technique pages, so it is worth learning before you read them.

The same convention numbers the units themselves. Rows are R1 through R9 from top to bottom; columns are C1 through C9 from left to right; boxes are 1 through 9 reading left to right then top to bottom (so box 1 is top-left, box 5 is the centre, box 9 is bottom-right). With this you can write a deduction unambiguously — for example, "5 is a hidden single in box 5 at r5c5." The notation page has the full convention with a labelled grid.

How difficulty levels actually work

Sudoku difficulty is not about bigger numbers — every puzzle uses the same digits and the same rules. It comes from two things at once: how many givens the puzzle starts with, and which solving techniques it requires. Fewer givens is one signal (easy puzzles ship around 36–45 of them; expert grids can ship as few as 22–24). The harder signal is whether you can finish using only the simple deductions, or whether you have to reach for advanced patterns.

On the 9×9 board we offer six bands. Easy puzzles fall to direct scanning and have lots of givens. Medium puzzles need basic candidate tracking and hidden singles. Hard puzzles require naked or hidden pairs and locked candidates. Expert calls for fish like the X-Wing and wing patterns. Master grids need chains and colouring. Evil is the hardest band, with the fewest givens and the deepest logic. The 6×6 mini grid (2×3 boxes, digits 1–6) stops at hard — three levels total. The 4×4 kids grid (2×2 boxes, digits 1–4) ships a single easy level for first-time players. The difficulty levels page lists them all exactly.

One thing worth knowing early: clue placement matters as much as clue count. Two puzzles with the same number of givens can feel very different, because where the givens sit affects which deductions are available and in what order. That is also why a "proper" sudoku always has exactly one solution — see the unique solution page for why that guarantee is the whole point.

• 9×9 — six bands: easy, medium, hard, expert, master, evil.
• 6×6 mini — three bands: easy, medium, hard.
• 4×4 kids — one band: easy.
• Fewer givens and harder required deductions both push a puzzle up the ladder.

The reading order through the ten basics topics

The ten topics below are arranged in a deliberate reading order — orientation first, then rules, then anatomy, notation, grading, construction, and finally a guided first solve. If you are completely new, read them top to bottom. Each one is a short page (three to five minutes), so the whole set takes about forty minutes end to end. By the bottom of the list you will have solved an easy puzzle yourself.

If you already know how the grid works and want a specific gap filled, jump in. The sudoku rules page is the learner-friendly rules statement; the pencil marks page teaches Snyder notation; the difficulty levels page explains what to expect at each level on each grid size. Anything that needs a name on this page is its own basics topic below — the essay positions them, the cards have the detail.

• 1. What Is Sudoku? — the puzzle in one minute.
• 2. The Rules of Sudoku — the rules statement in plain language.
• 3. Anatomy of a Sudoku Grid — cells, rows, columns, boxes, peers, bands, stacks.
• 4. Givens and Candidates — fixed clues vs digits still legal.
• 5. Pencil Marks and Snyder Notation — full marks vs the sparse Snyder style.
• 6. Sudoku Notation — rNcN cell coordinates and box numbering.
• 7. Sudoku Difficulty Levels — the bands per grid size.
• 8. Why a Sudoku Has One Solution — the uniqueness guarantee.
• 9. How Sudoku Puzzles Are Made — generation and clue removal.
• 10. Your First Sudoku: A Walkthrough — a real puzzle, step by step.

When you are done with the basics

You are done with the basics when you can name the parts of the grid, you understand what a candidate is, you know whether to write pencil marks in full or in Snyder style, and you have finished at least one easy puzzle in the first puzzle walkthrough. That is a low bar — and it is enough to start the next thing.

The next thing is the naked single. It is the simplest solving technique: a cell where only one candidate is still legal, so the surviving digit is forced. Pair it with the hidden single (a digit that has only one cell left in a unit) and you can finish almost every easy puzzle and most medium ones. After those, how to play covers the rules in their full pillar form and the solving order guide walks you through which technique to try in what order. The basics get you ready; the techniques get you solving harder grids.