Expert Sudoku
This is the hardest grade the generator will produce, and it is hard in a specific way rather than a vague one. Somewhere in the board there is a position that no pair and no single will break, and the way through is a group of three cells or a digit confined to one line of a box. Every puzzle on this page has been solved by the engine's own technique list before being handed over, so a route through exists.
The naked triple does not need three cells with three candidates each
This is the point most explanations skate over. A naked triple is three cells in one unit whose candidates, taken together, come to exactly three digits. They do not each have to hold all three. Cells reading {1,2}, {2,3} and {1,3} are a perfectly good triple on 1, 2 and 3: whichever way the digits fall, those three cells will use up all three numbers, so 1, 2 and 3 can be crossed out of every other cell in the unit. Because two of the three cells look like ordinary pairs, the triple is easy to walk past, and walking past it is what makes the grade feel impossible.
The other deduction here is a pointing pair turned ninety degrees
Worth being exact about, because this is the one place the grading on this site is arbitrary and pretending otherwise would be dishonest. A digit confined to a single row inside a box has to be somewhere in that stretch of row, so it leaves the rest of the row: that is the pointing pair, and it is what the hard grade is named for. A digit confined to a single column inside a box leaves the rest of that column, six cells in two other boxes, and the board labels that hint box line.
They are one argument with the page turned sideways, and neither is harder to spot than the other. The grader puts the column form one rung above the row form because the two sit next to each other on a fixed ladder and something had to come first – not because a solver finds it more difficult. So a board that reached this page on box line alone is a hard board that happened to be confined the other way round, and it is the naked triple above that genuinely earns the label. If a puzzle here collapses quickly, look at which technique the hints named.
No board on this page needs a guess
The generator does not simply remove digits until the puzzle looks difficult. After every single removal it re-solves the whole board with the technique list a person would use, and if that solver cannot finish, the digit goes straight back. The puzzle that reaches you is therefore one its own solver completed without ever branching. There is a real distinction here: a grid that has exactly one solution can still be impossible to reason through, and the only way to finish it is to pick a candidate, follow it until it contradicts itself, and back out. That is a search, not a puzzle, and it is filtered out before the board is drawn.
The sudoku expert label is a ceiling, not a score
The label describes the hardest rung the board forced, not how many times it forced it. A puzzle that demands one triple in the whole grid and a puzzle that demands four both come out at this grade, and they will not feel the same to solve. Difficulty measured by technique is honest about what a puzzle needs and silent about how often it needs it, which is worth knowing when two boards from the same page take four minutes and forty.
Working method for this grade
Scanning alone will not finish these. The usual approach is to place everything that falls to singles first, then switch the Notes button on and pencil candidates into what is left, and only then go looking for groups. Keep the marks honest: a triple spotted from a cell whose candidate list is out of date is worse than no deduction at all, because it removes a digit that was still viable and the board becomes unsolvable without ever looking wrong. If that happens, Check will report a count of wrong cells, and the honest response is usually to start the grid again rather than hunt for the fork.