Sudoku glossary: every term a hint or a lesson uses
31 Sudoku terms defined in plain language, each with a one-line example. These are the same definitions the app shows when you tap a word inside a hint, so the vocabulary on this site, in the lessons and in the game is one vocabulary.
Grid
The whole board: 81 cells laid out as 9 rows, 9 columns and 9 boxes.
The whole board is the grid. Every row, column and box is a part of it.
Cell
One square of the grid. A cell holds a single number, or your small notes while you are still deciding.
The highlighted square is one cell. Every number you write goes into a cell.
Row
Nine cells side by side, going across the grid. Each row holds the numbers 1 to 9 exactly once.
These nine cells across the grid are one row. There are 9 rows on the board.
Column
Nine cells stacked from top to bottom. Each column holds the numbers 1 to 9 exactly once.
These nine cells from top to bottom are one column. There are 9 columns on the board.
Box
A block of 3×3 cells inside the thicker lines. Each box holds the numbers 1 to 9 exactly once.
These nine cells inside the thick lines are one box. There are 9 boxes on the board.
Candidate
A number that could still go in a cell, shown as a small pencil mark (also called a note).
The small pencil marks in a cell are its candidates. Here 2, 5 and 7 can still go in this cell.
Unit
A row, a column, or a 3×3 box: any group of nine cells that must hold the numbers 1 to 9 exactly once. Other Sudoku sites sometimes call it a house or a region.
A unit is one full row, one full column, or one 3×3 box. Here the top row and the bottom-left box are both units.
Given
A number that is already on the board when the puzzle starts. Givens are locked: they cannot be changed or erased.
The bold numbers came with the puzzle. They are the givens, and they stay where they are.
Note
A small number you pencil into a cell to remember what could still go there. Notes are reminders, not answers, and you can add or remove them freely.
The small 2, 5 and 7 are notes. The big 4 next to them is a number that has been placed.
See / sees
Two cells see each other when they share a row, column, or box, so they cannot hold the same number.
These two cells see each other because they share a row, so they cannot hold the same number.
Line
A row or a column. Techniques that work the same way in both directions talk about lines instead of naming one.
This row is a line, and so is this column. Both are nine cells in a straight run.
Crossing
The three cells a box and a line share. A number placed there sits in the box and in the line at the same time. In fish patterns like X-Wing, the crossing lines are the ones that run across the base lines.
These three cells belong to the outlined box and to the outlined row at once. That overlap is the crossing.
Corner
One of the cells at the ends of a rectangular pattern, like the four cells that carry the candidate in an X-Wing.
The candidate 4 sits at these four cells, the corners of the rectangle the pattern is built on.
Bivalue cell
A cell with exactly two candidates left.
A bivalue cell has only two candidates left. Here only 4 or 9 can still go in it.
Strong link
A link between two candidate possibilities: if one is false, the other must be true. A common example is a number that fits in only two cells of a unit, called a conjugate pair. The two candidates in a bivalue cell also form a strong link, as can a single cell and a group of cells.
In this column the number 3 fits in only two cells, so one of them must be 3. That pair is a strong link.
Pivot
The hinge cell of a wing pattern that sees its wing cells: two in a Y-Wing or XYZ-Wing, and three in a WXYZ-Wing.
The green cell in the middle is the pivot. It sees both blue wing cells.
Wing
One of the two outer cells, or outer sets, of a wing pattern: the two ends that the middle of the pattern links together. In a Y-Wing or XYZ-Wing they are the bivalue cells the pivot sees. X-Wing borrows the name for its shape alone, and has four corners rather than wing cells.
The two green cells at the ends are the wings, and the pivot between them sees both.
Hinge
The cell or set in the middle of a pattern that touches both wings and carries the logic from one to the other.
The set in the middle is the hinge. It links the wing set above it to the wing set below it.
String
One of the two strong links of a Two-String Kite, one running along a row and one along a column, meeting in a shared box.
The row link and the column link are the two strings. They meet inside the outlined box.
Fin
An extra candidate occurrence on a base line outside a fish pattern's cover lines. It prevents the ordinary fish elimination, but candidates on the cover lines outside the base lines can still be removed if they see every fin.
This almost forms an X-Wing on 4. The extra blue cell, with 4 as a candidate, is the fin.
Roof and floor
Roof and floor are paired terms used in two techniques. In a Skyscraper the floor is the two strong-link ends that share a row or column, and the roof is the other two ends. In a Unique Rectangle the floor cells hold only the shared pair, and the roof is where the extra candidates that would break the deadly pattern appear. Either way, the elimination is driven by the roof cells.
The two blue cells at the bottom share a row, so they are the floor. The two green cells at the other ends are the roof.
Cell group
Two or three cells of one box that also share a row or a column. A number confined to them behaves like a single cell, which is what lets grouped patterns treat them as one.
These two cells lie in the same box and the same row, so the pattern can treat them as one group.
Set
A handful of cells looked at together. If the cells all share a row, column or box and hold exactly as many different candidates as there are cells, they form a locked set: those digits fill the set and can be removed from the other cells in any row, column or box shared by all cells in the set.
These three cells hold only 2, 6 and 8 between them, so together they form a locked set.
Base
The rows or columns where a fish pattern's candidate is restricted: 2 lines for an X-Wing, 3 for a Swordfish.
In these two outlined rows, the number 4 appears only in the marked cells. These rows are the base.
Chain
A series of linked cells. Following the links shows how one candidate being true or false forces the next candidate.
The lines link the cells into a chain, stepping from one cell to the next.
Color
One of the two labels painted along a chain of strong links. Exactly one color is true and the other is false, so a candidate that clashes with both can be removed.
This chain is painted in two colors. One of them is true, and we do not yet know which.
Deadly pattern
Four unsolved cells at the corners of a rectangle in two rows, two columns, and two boxes, each limited to the same two candidates. The two digits could be swapped within those four cells for a second solution, so this pattern must be avoided.
These four corner cells all have the same two candidates, 3 and 7. That is a deadly pattern a valid puzzle never allows.
Almost Locked Set
A set of cells all in one row, column, or box, with one more distinct candidate than there are cells. If one of those candidates is ruled out from the whole group, the remaining candidates in those cells form a locked set. Short: ALS.
These two cells have three candidates between them, 3, 5 and 7, one more than the number of cells. That makes an Almost Locked Set.
Restricted Common Candidate
A candidate shared by two sets where every cell with it in one set sees every cell with it in the other, so that digit can be placed in at most one of the two sets. Short: RCC.
The candidate 5 is shared by the green set and the blue set, and the two 5s see each other. That makes 5 the restricted common candidate.
Helper cell
In Sue de Coq, one of the two cells outside the crossing, one in the line and one in the box, that take their two candidates from the same pool as the crossing cells.
This cell sits outside the crossing but takes its two candidates from the same pool. It is a helper.
Pool
All the distinct candidates found across a group of cells. The pool can contain more digits than there are cells, so those cells do not necessarily use every digit in it.
These three cells must place 3, 5 and 7 between them. Those three digits are their pool.