Hidden Pair

A hidden pair occurs when, in a house (row, column, or box), two digits can appear in only two cells, while those two cells still contain other candidates. In that case, all other candidates can be eliminated from those two cells. No matter how the digits are arranged, they must occupy those two cells.

Hidden Pair is a size-$2$ subset.

A hidden pair can appear in a row. For example, in row $4$ below, digits $1$ and $5$ can appear only in $R4C2$ and $R4C4$, so all other candidates can be eliminated from those two cells.

A hidden pair can also appear in a column. For example, in column $6$ below, digits $4$ and $7$ can appear only in $R4C6$ and $R6C6$, so all other candidates can be eliminated from those two cells.

A hidden pair can also appear in a box. For example, in box $9$ below, digits $3$ and $6$ can appear only in $R8C8$ and $R9C7$, so all other candidates can be eliminated from those two cells.