Last Wednesday was September 9. The World Puzzle Federation settled on that date in 2013, the ninth day of the ninth month, after the nine by nine grid. It is the one date in the year that belongs to the puzzle.
International Sudoku Day, No. 65
A Wednesday normally prints a Medium. September 9 printed a Master, numbered No. 65 and named for the day.
It came out with 24 given digits and 57 empty squares. Here is the part you would have no way to know from looking at it. No. 65 never needs a technique past the White Belt. It never needs a pair and never needs a pointing pair. All 57 squares come out with full houses and singles, and 26 of them are hidden singles, a digit that fits in only one square of its row, column, or box.
That is what a grid like that asks of you. You follow one digit at a time across a board with almost nothing written on it, you place what is proven, and you do it 57 times. Patience is the whole technique.
A box points to a line
The pointing pair is the first move you cannot see without pencil marks, which is why the marks arrive when it does.
Take one box and one digit. Mark every square in the box where that digit can still go. If all of those squares sit on the same row, the box has to place the digit somewhere on that row, and every other square in the row loses it. The same holds for a column.
Here is a real position from Saturday's Master, No. 68, with 38 digits on the board.
Look at the top right box, rows 1 to 3 and columns 7 to 9. Five squares are filled: a 1 at row 1 column 9, a 7 at row 2 column 8, and a 5, an 8 and a 2 along row 3. Four are empty: row 1 columns 7 and 8, row 2 columns 7 and 9.
Where can a 3 go in that box? Row 1 already holds a 3, at column 2, so neither of the row 1 squares can take one. That leaves row 2 column 7 and row 2 column 9. Both of them sit in row 2.
The box has to put its 3 in one of those two squares. So the 3 in row 2 is inside that box, and the rest of row 2 cannot hold one. Row 2 column 4 was showing 3, 4 and 8. Remove the 3 and it is down to 4 and 8.
The 4 works the same way. Row 1 holds a 4 at column 1, so the box's 4 also has to sit at row 2 column 7 or row 2 column 9. Remove 4 from the rest of row 2. Row 2 column 4 loses its 4 and is left with 8 alone. Row 2 column 6 was showing 4, 8 and 9, and comes down to 8 and 9.
Row 2 column 4 is an 8, and it was certain before either deduction placed a digit. The eliminations did all of the work. Row 2 column 6 follows as a 9.
The usual mistake is to clear the digit out of the rest of the box. Nothing inside the box has been proven. The box is only where you found the pattern. Every elimination happens on the line, outside the box, and the two squares of the pair keep every candidate they had.
The week's editions
Seven printed: Easy No. 63 and No. 64 on Monday and Tuesday, the International Sudoku Day Master No. 65 on Wednesday, Medium No. 66 on Thursday, Hard No. 67 on Friday, Master No. 68 on Saturday, and Expert No. 69 on Sunday.
No. 68 was the only one of the seven that needed a pointing pair, and it needed two, the two you have just worked through. The other six came out with full houses and singles, the Friday Hard and the Sunday Expert among them.
The number of givens sets the level printed at the top of the page. What a grid asks of you is a separate question, and it is worth noticing which one you are up against.
In Claire's Sudoku
The pointing pair is the first lesson of the Yellow Belt, and it is the lesson where the app hands you pencil marks. It walks one position with you. The drill then gives you five positions to find from the techniques you have been taught, about a minute in all, and it leans on the ones you have been getting wrong.
If you take one thing into this week, open the Yellow Belt lesson in Claire's Sudoku and work it through once. The next time a box confines a digit to a single line, you will see it.
The nineteenth World Sudoku Championship runs October 11 to 15 in Kolkata, hosted by Logic Masters India. I will write about it once the results are in.