These are the patterns our puzzles are built on, in the order you will need them. Easy puzzles need only the first two. Expert puzzles need all of them.
01
A cell where eight of the nine numbers are already used in its row, column and box. The ninth number is the only one left, so it goes there.
Needed from Easy
03
Inside a box, all the places for a number lie on one row or column. The number must end up on that line within this box, so you can remove it from the rest of the line.
Needed from Medium
04
The reverse of a pointing pair. In a row or column, all the places for a number lie inside one box. Remove that number from the other cells of the box.
Needed from Medium
05
Two cells in the same unit with the same two candidates, for example 3 and 8. One of them gets the 3, the other the 8. Neither number can go anywhere else in that unit.
Needed from Hard
07
Three cells in a unit whose candidates together are only three numbers. Those three numbers are spoken for, so remove them from the rest of the unit.
Needed from Hard
08
Pick a number. If in two rows it can only go in the same two columns, the four cells form a rectangle. The number takes two opposite corners, so remove it from the rest of both columns. It works with rows and columns swapped too.
Needed from Expert
09
A pivot cell with candidates X and Y sees two wing cells: one with X and Z, one with Y and Z. Whatever the pivot becomes, one wing turns into Z. So any cell that sees both wings cannot be Z.
Needed from Expert
10
The X-Wing on three lines. If a number fits in three rows only within the same three columns, remove it from the rest of those columns.
Needed from Expert