Sudoku glossary
These are the terms you will meet in variant sudoku rules, solving videos and puzzle comments. Some name a type of marking on the grid, some name a solving technique and some name a way of taking notes. Each definition links to the lessons where the term matters most. Setters don't always use words the same way, so when a puzzle's rules text defines a term, go with the puzzle.
- 45 rule
- The digits 1 to 9 add up to 45, so every row, column and box sums to 45. Compare that with the cage totals inside a house to find the sum of the cells left over. Killer cages lesson · Little killer lesson
- Anti-king
- Two cells a chess king's move apart, including diagonally, can't hold the same digit. In a sudoku, the only new pairs this adds are diagonal neighbors in different boxes. Anti-king lesson
- Anti-knight
- Two cells a chess knight's move apart (2 steps one way and 1 step the other) can't hold the same digit. Anti-knight lesson
- Arrow circle
- The circle at the start of an arrow. The digits along the arrow add up to the digit in the circle. Arrows lesson
- Arrow pill
- A long oval covering 2 or 3 cells at the start of an arrow. Its digits are read as one number, such as 1 and 4 making 14, and the arrow's digits add up to that number. Arrows lesson
- Between line
- A line with a circle at each end. Every digit on the line is strictly greater than one circle and strictly less than the other. Between lines lesson
- Bifurcation
- Trying one option in a cell and following it to see if it breaks. Many solvers treat it as a last resort, and well-made puzzles can be solved without it. Pencil marks lesson
- Bivalue cell
- A cell with exactly two candidates. These cells are the start of most pair and chain deductions. Pencil marks lesson · Classic sudoku lesson
- Black dot
- A black Kropki dot joins two digits where one is double the other, such as 3 and 6. The pair 1 and 2 fits both a white dot and a black dot. Kropki dots lesson
- Break-in
- The first deduction in a puzzle, often the hardest to find. Setters usually hide it where two rules meet. Classic sudoku lesson
- Cage
- A group of cells outlined with a dashed line. In most puzzles the digits in a cage can't repeat, and a clue in its corner gives their sum. Killer cages lesson
- Cage combination
- A set of distinct digits that fits a cage's size and total. Some totals allow only one set: 2 cells summing to 3 must be 1 and 2, and 2 cells summing to 17 must be 8 and 9. Killer cages lesson
- Candidate
- A digit that could still go in a cell. Solving is mostly the work of removing candidates until one is left. Pencil marks lesson
- Center mark
- Small digits in the middle of a cell. They list every digit the cell could still be. Pencil marks lesson
- Coloring
- Shading cells to track how they relate. One common use is to give two colors to two options and follow each until one leads to a clash. Pencil marks lesson
- Corner mark
- A small digit in the corner of a cell. It means this digit goes in one of the cells marked this way in the house. Pencil marks lesson
- Deadly pattern
- Four cells in two rows, two columns and two boxes that could all swap between the same two digits. That would give two solutions, so in a unique puzzle at least one of those cells must hold something else. Classic sudoku lesson
- Diagonal sudoku
- A puzzle where one or both main diagonals also hold 1 to 9 with no repeats. It is also called sudoku X. Diagonals lesson
- Disjoint subsets
- Cages in the same house can't share a digit. So their combinations must fit together without overlap, which often rules out options that look fine on their own. Killer cages lesson
- Dutch whispers
- A line where neighboring digits differ by at least 4. A 5 can go on it only if both its neighbors on the line are 1 or 9. Dutch whispers lesson
- Entropic line
- A line where any 3 cells in a row along it hold one low digit (1 to 3), one middle digit (4 to 6) and one high digit (7 to 9). The pattern repeats every 3 cells. Entropic lines lesson
- Extra region
- A shaded group of 9 cells that works like an extra box. The digits 1 to 9 appear in it once each. Extra regions lesson
- Fog of war
- Most of the grid starts hidden. Placing correct digits clears the fog around them and shows clues that were covered. Fog of war lesson
- German whispers
- A line, usually green, where neighboring digits differ by at least 5. A 5 can never go on it, and digits alternate between high (6 to 9) and low (1 to 4). German whispers lesson
- Givens
- The digits printed in the grid at the start. You can't change them. Many variant puzzles have very few givens, and some have none, because the extra rules carry the information instead. Classic sudoku lesson
- Global constraint
- A rule that applies everywhere in the grid with no markings, such as anti-knight or non-consecutive. You have to check it for every cell you fill. Anti-knight lesson · Anti-king lesson · Non-consecutive lesson
- Two digits that can only go in the same two cells of a house. Those cells must hold those digits, so any other candidates in them can go. Classic sudoku lesson · Pencil marks lesson
- A digit that has only one possible cell left in a house. The digit goes there, even if that cell has other candidates. Classic sudoku lesson
- House
- A row, column or 3x3 box. Each house holds 1 to 9 exactly once. Extra regions and marked diagonals work as extra houses. Classic sudoku lesson · Extra regions lesson · Diagonals lesson
- Innie
- A cell inside a house (or group of houses) that isn't covered by cages lying fully inside it. The 45 rule gives you its value or the sum of all such cells. Killer cages lesson
- Killer cage
- A cage with a sum clue. Its digits add up to the clue and don't repeat. Killer cages lesson
- Kropki dot
- A dot on the border between two cells. The color tells you how the two digits relate. White and black are the usual colors, but check the puzzle's rules. Kropki dots lesson
- Little killer
- A clue outside the grid with a small arrow pointing along a diagonal. The digits on that diagonal add up to the clue, and they may repeat unless another rule stops it. Little killer lesson
- Naked pair
- Two cells in one house that can each only be the same two digits. Those two digits must go in those two cells, so you can remove them from every other cell in the house. Classic sudoku lesson · Pencil marks lesson
- Naked single
- A cell with only one candidate left. Its row, column and box already rule out the other 8 digits. Classic sudoku lesson
- Naked triple
- Three cells in one house whose candidates, taken together, are only three digits. Those three digits are used up by those cells and can be removed from the rest of the house. Classic sudoku lesson · Pencil marks lesson
- Negative constraint
- A rule saying all possible clues of a type are shown. Where a dot, X or V is missing, the two cells do not have that relationship. Kropki dots lesson · XV lesson
- Non-consecutive
- Cells that share an edge can't hold digits that are one apart. A 5 can't sit beside a 4 or a 6. Non-consecutive lesson
- Odd and even cells
- Shaded shapes that fix a cell's parity. A gray circle usually means odd and a gray square usually means even, but check the puzzle's rules. Odd and even cells lesson
- Outie
- A cell outside a house that belongs to a cage sticking into it. Subtracting 45 from the total of the cages that cover the house gives the outies' sum. Killer cages lesson
- Palindrome line
- A line that reads the same from either end. Cells in matching spots hold the same digit, so they can never share a house. Palindromes lesson
- Parity
- Whether a digit is odd or even. The digits 1 to 9 include 5 odd digits and 4 even ones, which helps when counting what a house still needs. Odd and even cells lesson · Parity lines lesson
- Parity line
- A line where digits alternate between odd and even along its length. Parity lines lesson
- Pointing pair
- When a digit's only spots in a box all sit in one row or column, the digit must be in that box's part of the line. You can remove it from the rest of that row or column. Classic sudoku lesson
- Quadruple
- A circle on the corner where 4 cells meet, holding up to 4 digits. Each digit shown must appear in at least one of those 4 cells. Quadruples lesson
- Region sum line
- A line that crosses box borders. Each piece of the line inside one box adds up to the same total. Region sum lines lesson
- Renban
- A line, usually purple, holding a set of consecutive digits in any order with no repeats. A 4-cell renban could be 3, 6, 4, 5 but not 2, 3, 5, 6. Renban lines lesson
- Sandwich sum
- A clue outside the grid giving the sum of the digits between the 1 and the 9 in that row or column. A 0 means the 1 and 9 are next to each other, and 35 means they sit at the two ends. Sandwich sums lesson
- Sees
- One cell sees another when they can't hold the same digit, because they share a house or because a variant rule links them. Anti-knight and anti-king rules make cells see each other across the grid. Classic sudoku lesson · Anti-knight lesson · Anti-king lesson
- Set theory
- Comparing two groups of cells that must hold the same digits, such as a row and a matching set of cages. Cells that are in one group and not the other must hold the same digits as each other. Killer cages lesson · Region sum lines lesson
- Snyder notation
- A corner-marking habit: you mark a digit in a box only when it can go in just two cells there, sometimes three. It keeps notes short and makes pairs easy to spot. Pencil marks lesson
- Thermo bulb
- The round end of a thermometer. Digits rise from the bulb toward the other end, so the bulb holds the smallest digit on the thermo. Thermometers lesson
- Uniqueness
- A well-made puzzle has exactly one solution. Some solvers use this as a logic step, ruling out any pattern that would allow two solutions. Classic sudoku lesson
- White dot
- A white Kropki dot joins two digits that are one apart, such as 4 and 5. Kropki dots lesson
- X-sum
- A clue outside the grid. Read the first digit from that side (call it X), then add up the first X cells, including that first one. The total equals the clue. X-sums lesson
- X-wing
- A digit is limited to the same two columns in two different rows. The four cells form a rectangle, and the digit takes two opposite corners, so it can be removed from the rest of those two columns. The same works with rows and columns swapped. Classic sudoku lesson
- XV
- An X between two cells means they add to 10. A V means they add to 5. If the puzzle says all X and V clues are given, every other pair of neighbors adds to something else. XV lesson