zedoku

pointing pairs

level 3 - medium

also known as: pointing pair, box-line reduction

what it means

when two cells in a box share the same candidate number and are aligned in the same row or column, they point to eliminate that number from the rest of that row or column outside the box.

example 1: vertical pointing pair

look at this box. two cells in column 0 both have 8 as a candidate. they are aligned vertically and point down the column.

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the 8s in column 0 form a pointing pair

the boxthe pointing pair

example 2: before elimination

since one of the two cells must contain 8, no other cell in column 0 can have 8. the candidates that will be eliminated are crossed out.

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the 8s below will be eliminated

elimination zone (column)8candidates to be eliminated

example 3: after elimination

the 8s have been removed from the rest of column 0. notice how some cells now have only one candidate left!

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8 eliminated from the rest of the column

the cells at rows 6, 7, and 8 now have only one candidate each — they are now obvious singles!

key concept: the doorway analogy

imagine two people standing in a doorway, both pointing down a hallway. they are blocking the only entrance to that hallway.

  • the doorway is the 3×3 box
  • the two people are the cells with the same candidate
  • the hallway is the row or column they are aligned in
  • their pointing blocks that number from entering the rest of the hallway!

since one of those two cells must contain that number, no other cell in that row or column outside the box can have it.

example 4: horizontal pointing pair

pointing pairs can also be horizontal. here, two cells in row 4 both have 8 as a candidate, pointing across to the right.

the pair

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8s in row 4, columns 4-5

before elimination

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8s in the row will be eliminated

the two 8s in the middle box point horizontally. since one of them must be 8, all other 8s in row 4 outside the box can be eliminated.

after horizontal elimination

after removing 8 from the rest of the row, several cells are simplified:

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8 eliminated from the rest of the row

multiple cells now have fewer candidates, making the puzzle easier to solve!

visual summary

vertical pointing

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pair in column → eliminates below

horizontal pointing

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pair in row → eliminates across

summary

  • find a box where the same candidate appears in two cells aligned in a row or column
  • those two cells form a pointing pair
  • since one must contain that number, eliminate it from the rest of that row or column
  • this often reveals new singles and simplifies the puzzle
  • works both vertically (pointing down/up) and horizontally (pointing left/right)
← back to tutorialnext: obvious triples →