Sudoku Tutorial
Empty Rectangle (hinge)
An Empty Rectangle is also known as a Hinge. In this example the hinge is in box 2.  It allows the blue cell in row 1 to bend around the corner and 'see' the  target cell.

Here's how it works:
     The two blue cells have a strongly linked candidate 1.   The target in this case is r9c4.  

One of the blue cells must be a 1.  Obviously if the bottom blue cell is a 1, the slashed 1 can be eliminated.

The upper blue cell can indirectly 'see' the target cell (r9c4) via the 'hinge'.  If the top blue cell is a 1, the other 1's in row one would be eliminated, causing the 1 in r3c4 to be a 1 (it would be a hidden single in box 2).  That would eliminate the slashed 1 (r9c4).

So either way, the slashed 1 can be removed.

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