When every cell a box still has open to a digit lies on one row or one column, the box's copy of that digit is on that line, so every other cell of the line loses it. Two cells make a pointing pair, three make a pointing triple.
How to spot it
Pick a box and a digit the box is still missing, and ask where that digit can go. It is the question the hidden single asks. When the answer is one cell, you have a placement. When it is two or three cells, look at where they sit.
A box is three rows by three columns. If every cell still open to the digit is in the same row of the box, or the same column, the box has settled which line its copy goes on without settling which cell. That line carries on out of the box and through two more boxes, and it can hold the digit only once. The box is going to use that one, so every other cell of the line loses the digit.
Two cells make a pointing pair and three make a pointing triple. There is no pointing quad, because a row or column of a box has only three cells.
Ask box two where the 7 can go
Box two has four empty cells. The centre one cannot take a 7, because row two already has one at its far end. The bottom-right one cannot either, because column six has a 7 further down, in row eight. That leaves the left and middle cells of the box's top row.
Both cells are in row one
One of those two cells will hold the 7. You cannot tell which yet, and you do not need to. Both are in row one, so wherever it lands, box two's 7 is in row one.
Follow the row out of the box
Row one can hold one 7, and box two is going to supply it. So no other cell of row one can be a 7, whatever else it shows. Three cells outside the box still carry a 7, all of them in box one, and all three lose it. Nothing inside box two changes.
Read what the strike left behind
The first cell of row one was showing 3 and 7. Without the 7 it has one candidate left, so it is a 3: a naked single. Before the strike this grid had no single of either kind anywhere. The pair is what moved it.
Every mark in the box counts
Pointing is about one digit, not about two cells. The two cells that just gave up their 7 show 2, 3, 6 and 7 each, and only the 7 points.
Take the 6. It sits in those two cells and also in the centre cell of the box, which is in row two. So the box has not settled a line for its 6. Strike it from row one anyway and it comes out of the seventh cell. On this grid the seventh cell of row one is where the 6 belongs, so that strike leaves the puzzle with no solution at all.
Before you strike, find the digit in every empty cell of the box, including the ones away from the pair.
Both of the box's 7s are in row one. The rest of row one loses its 7s.
A third 6 sits in the centre cell, in row two. Row one keeps its 6.
Two cells or three
A pointing triple is the same move with three cells. The digit's homes fill a whole row or a whole column of the box, and the argument does not change: the box's copy is on that line, so the rest of the line loses it.
Box eight has one. Its 8 can only go in the three cells of its left column, which is column four, because columns five and six each hold an 8 higher up. So box eight's 8 is in column four. Column four has nothing to give up, though. Its other three empty cells, in rows one, four and five, were already closed to the 8. The triple is real and removes nothing, which happens, so check the line before you count on a result. The question at the end of this page has a triple that does remove something.
The strikes land on the line, outside the box. If you find yourself removing a digit inside the box, you are running the move backwards. That direction exists too, a row or column whose digit is confined to one box clearing the rest of that box, and it is the next technique on the path, the claiming group.
Where this sits on the path
The three techniques before this one on the path all place a digit. Pointing places nothing. It only removes candidates, and it works for the singles rather than replacing them. On the grid above the strike placed nothing by itself. It turned the first cell of row one into a single.
That 3 was out of reach of the last possible number. The cell's row, column and box rule out seven digits between them. The eighth, the 7, was ruled out by the pair, and no placed digit shows that. Once candidates are coming out, pencil marks start to see singles a bare grid cannot.
The subsets chapter carries the same idea further. A naked pair ties two digits to two cells, and like pointing it changes cells other than the ones you found it in. A hidden pair ties them from the digits' side. Pointing is the smallest version of the idea: one digit, tied to one line of one box.
Row five runs through box five. Which candidate can you strike from the third cell of row five?
Take each digit that cell shows and ask box five where that digit can go.
Recap
- Look for
- A box where every cell still open to one digit is on the same row or the same column.
- Scope
- Starts in a box and removes along a row or column. Two cells make a pair, three make a triple.
- Result
- The digit comes out of every cell of that line outside the box. Nothing inside the box changes.
- Tell-tale
- Ask a box where a digit can go. Two or three answers in a line, and none anywhere else in the box.
- If it fails
- If one more cell in the box carries the digit off the line, the box has not settled a line and nothing can be struck. Try the next digit or the next box. If no box points, turn the question round with the claiming group, or look for a naked pair.