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Empty Rectangles
 Empty Rectangles constitute a clever but obscure strategy for Sudoku. An Empty Rectangle occurs within a box where four cells form a rectangle which does NOT contain a certain candidate.In the first diagram is a spread of candidate 8 around the board but each box must be considered separately when imagining the rectangle, so I have coloured the boxes differently. To be able to apply this strategy we must have at least two candidates left in the box and four or more empty cells.The red crosses represent the Empty Rectangles. Each of these theoretical rectangles is independent of the others. Empty Rectangles in Boxes
 Whatever the formation of the Empty Rectangle (ER), we will have a vertical and horizontal gap where we can imagine two Empty Rectangle Lines (ERLs). These are two lines we can draw inside the box without touching the ER itself.I have picked box two in this example. Empty Rectangle Lines
 We can quickly forget about these lines once we've spotted where they cross - a place known as the Empty Rectangle Intersection (ERI).In this diagram the ERIs have been marked with a brown cell. Empty Rectangle Intersections
 If you have a strong link and an ERI that shares a line (row or column), then you can eliminate a candidate at that point where the other end of the strong link and the ERI intersect. Let's look at some visual examples of the formation. Two different arrangements are shown in this and the next diagram.The plus sign now indicates where the Empty Rectangle Intersection (ERI) is placed. In the first example down column 2, two 8s exist and form a strong link (X and Y). One end of the strong link (X at C2) can see the ERI at C8. Example ERI arrangement
 In the second example the ERI at A7 can see an 8 in A4 (the X) and this is paired with the 8 on the cell marked Y. Y can see the 8 in Z which in turn is visible to the original ERI. It is proposed in both examples that the 8 in Z can be removed. Example ERI arrangement
 Here is a real example based on the centre box. The remaining options for 6 in box 5 is D4, D5, E6 and F6. This gives us an Empty Rectangle shown by the four yellow cells. The Empty Rectangle Intersection is the orange cell on D6. Our X and Y are the two green cells H6 and H9. It doesn't matter that there are a ton of 6s in column 6, what is important is there are two 6s in row H. H6 can see the ERI and H9 can see the intersection of H9 and the ERI - which is D9. That is the 6 we can remove. Empty Rectangle Example: Load Example or : From the Start
 Here is a fascinating double Empty Rectangle. The ERI cell on E3 can see two strong links on candidate 6. There is one in row G between G3 and G9 and in the other in column 8 between E8 and H8. The first one allows us to remove 6 from E9 and the second from H3. Double the fun! Double Empty Rectangle: Load Example or : From the Start
 This awesome sudoku has great examples of certain strategies including a symetrical Hidden Quad. At this stage there is a perfectly centered Empty Rectangle based on candidate 3 with no clues in the rectangle cells. I have Klaus Brenner to thank for sending me this puzzle. Centered Empty Rectangle: Load Example or : From the Start
 Klaus Brenner has found some very nice looking triple Empty Rectangles. I didn't think we'd find any beyond the double one above, but no, here is one of five he has made. The solver normally returns just the first instance of an advanced strategy but in the case of Empty Rectangles it will return all instances. Klaus also found a puzzle that returns three different Empty Rectangles in a single step. You can load the puzzle from this link. Triple Empty Rectangle: Load Example or : From the Start

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Comments here pertain to corrections to the text, not the subject itself

## Friday 2-Aug-2013

### ... by: Civiliza

The Triple Empty Rectangle picture incorrectly uses a blue line to indicate a stong link between the 3's in E3 / E7 and another to indicate a strong link between the 7's in J3 / J7. The two strong links that should be joined with blue lines are the 7's in E3 / J3 and the 3's in E7 / J7.

Andrew Stuart writes:

Well spotted, I had a dizzy moment then, creating that image. Fixed now

## Tuesday 29-Jan-2013

### ... by: Dazdingoz0r

Hey. This is my first comment here and I`d like to take this chance to thank you this site that has helped me immensely.

I have recently started learning the strategies and couldn`t help but notice that here, in the 9th example [ 4th real example ], you made the strong links between 3s and 7s horizontally when in fact, the connections should be vertically, since there is a third 3 on row E at E2 [ so it`s not a strong link even with the elimination of the 3 candidate in E1 ] and two more 7s at J2 and J9 on the J row.

Vertically though, there are only two of each respectively on the 3rd and 7th columns, so that`s how the strong links should be drawn in the example.
I know it`s still possible to see how those two are connected eventually, with the colored boxes and all, but it may prove a bit difficult for someone who is just learning these to see the pattern, since he`ll likely try to make sense of the connections in the example.

Andrew Stuart writes:

Fixed. Doh!