When someone asks what separates the occasional Sudoku player from the one who finishes expert puzzles without a hint, the answer is one word: pencil marks. The occasional player looks at the board as it appears. The serious player fills the board with small numbers before making any real move.
Pencil marks are the external memory of Sudoku reasoning. The human brain cannot hold, simultaneously, every possible candidate for every empty cell. On easy puzzles, it does not matter. On medium and above, it matters a lot. Each empty cell can have between two and eight possible candidates. Multiply that by 50 or 60 empty cells and you get a matrix no one memorizes. Pencil marks are where that matrix lives.
The technique is simple. For each empty cell, write small the numbers from 1 to 9 that are still candidates, after discarding the ones already present in the same row, column and block. In Sudoku BLA, the notes mode is one tap away, and there is an auto-notes function that fills all that automatically, so you do not lose five minutes at the start of every puzzle.
With the notes in place, three things change. First: deduction techniques start to work. Without notes, naked pair is invisible, pointing pair too, X-wing as well. Techniques only show up when you have the candidate map in front of your eyes. Second: each new number entered on the board eliminates candidates across several cells at once, and the cascade effect becomes visible. Third: you do not forget what you have already deduced. The notes work like margin notes in a technical book.
A design decision in Sudoku BLA: notes appear in Sombra BLA color, a discreet warm gray (#8a8680). The first version of the app used the brand green for notes, but it became tiring to the eyes on long puzzles. Sombra is lighter, reads faster, visually separates the answer number (editorial black) from the candidate number (warm gray). Small detail, big effect in an hour-long session.
For someone leaving the easy level, the recommendation is: turn on auto-notes, play a few medium puzzles paying attention to how the candidate matrix updates, then start turning auto-notes off and marking the candidates yourself when you feel comfortable. In two weeks, taking notes becomes reflex. And then the hard level stops being intimidating. It just becomes the next puzzle.
