The method Hidden Single is similar to a Naked Singles in the sense that a given cell will have only one
possible value. The elimination is done differently. A collection of cells with the same number will force
(pin down) the number in a given cell.

The following example illustrates a Hidden Single in **B2** with respect to row **B**.
Consider row **B** and the cells with value **9** in blue. There cannot be a **9** in cells
**B1**, **B4**, **B6** and **B8**, because the blue cells contain a **9** and are aligned
in columns **1**, **4**, **6** and **9**. Hence, the only
possible place for a **9** in row **B** is in **B2**. Note that if we just focus on the possible
values of a given cell (as in Naked Singles), then
**B2** can be either **6** or **9** and both will be possible.
The Hidden Single elimination considers the positions of
all (or some) cells with value **9** and not just the possibilities for a given cell.

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In the previous example we looked at a Hidden Single in a row. In fact we can do this for columns and
squares in the same way. Here is an example where we consider the top-left square. The number **9** cannot
be placed in **A2**, **B2** or **C2** because of the blue **G2** cell with value **9**. Hence the
only possibility for **C1** is **9**.

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If you have the candidates option turned on, a Hidden Single can easily be found as a cell where a given
candidate appears only once in its row, column or square. In this example, candidate **8** in
cell **C5** appears only once in column **5**. Therefore, it is a Hidden Single and **C5**
has value **8**.

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