Check each position
Compare the same position in A and B. Union keeps a mark in either pattern, including their overlap. Intersection keeps only the overlap. A minus B keeps marks in A that are absent from B. Exactly one, also called XOR, keeps marks in either pattern but removes the overlap.
Remove the overlap
A has marks at the top left and top right. B has marks at the top right and bottom right. For exactly one, remove their shared top-right mark. The result keeps the top left and bottom right. Union would keep all three positions, so the overlap distinguishes the rules.
Predict before choosing
Now A has marks at the top and right. B has marks at the right and bottom. Apply exactly one. In a matrix, check the proposed rule against the complete rows or columns before filling the gap.
Try it
Which marks remain under exactly one?
- Top and bottom
- Right only
Show the answer
Top and bottom
Yes. The right mark is shared, so it disappears. The top and bottom each occur once.
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