Solving techniques
From easiest to most advanced. Easy puzzles only need the first five; expert and harder require techniques from the bottom of the list.
Last Free Cell
The simplest technique. When a row, column or 3×3 box has only one empty cell, you can immediately fill in the missing digit, just check which of 1–9 is absent.
Last Remaining Position
Looking at it from a digit's angle: pick one (say 5), see which boxes already have it. Check if in a neighboring box it can only go in one cell, usually it can.
Last Possible Number
Focus on one cell. Check 1–9 to see which can go there, every other is already in its peers (row/column/box). The one that fits is the answer.
Notes (candidates)
When uncertain, mark 2–3 possible digits as notes in the cell. As you fill neighboring cells, some candidates get eliminated and the answer reveals itself. Most medium puzzles already require notes.
Naked Single, only candidate
A cell where only one digit can go, because every other digit is blocked by its peers (row, column and box). Simplest technique, often unlocks chains of further moves.
In the highlighted cell only digits that are not yet in its row, column or box can appear. Checking 1–9 we find only one fits.
Naked Pair, pair in cells
If two cells in the same row, column or box can only contain the same two digits (e.g. only {3,7}), those digits "reserve" those cells. You can cross out 3 and 7 from notes of all other cells in that unit.
Two cells in one row have notes {3,7}. One of them will be 3, the other 7 (we don't know which). Means: in the remaining cells of this row there will never be a 3 or a 7.
Naked Triple, triple in cells
Extension of Naked Pair, three cells in the same unit contain only three same digits (e.g. {2,5,7}). Those digits "reserve" those cells. You can cross out 2, 5 and 7 from notes of remaining cells in that unit.
Pointing Pair, pointing duo
When in a 3×3 box a digit can only go in 2 cells (or 3) and all of them are in the same row or column, you can cross that digit out of all other cells in that row/column.
The 4 in the top-left box can only be in cells (0,1) and (0,2). Both are in row 0. Means: in row 0 the four is somewhere in this box, strike it from the rest of row 0.
Pointing Triple
Same as Pointing Pair but for three cells. A digit in a box can only go in three cells, all in the same row or column. Cross that digit out of the rest of the row/column outside this box.
X-Wing
If in two rows the same digit can only appear in two specific columns (forming a square, "wings"), that digit must be in those columns and those rows. You can cross it out of other cells of those columns.
Y-Wing
Three cells in a Y formation. The "pivot" has notes {A,B}, one "wing" has {A,C}, the other has {B,C}. Whatever ends up in the pivot, one of the wings must be C, so any cell seeing both wings cannot be C.
Swordfish
X-Wing version for three rows and three columns. If a digit in three rows can only be in the same three columns, you can cross it out of other cells of those columns. Rarely needed, mostly master/extreme levels.
Time to practice
You learn best by playing. Every new puzzle is a chance to try the techniques you just read about. The Hint button in the game will show you which technique to apply.
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