Logic grids: carrying a match across blocks
A ● says two items belong to one person. So every × and ● in one item’s row is true for the other. Copy them across.
How it works
In a logic grid, a ● between Ada and the beagle means Ada is the beagle owner. They are one person with two names.
So anything true of Ada is true of the beagle owner. If Ada didn’t adopt in 2019, neither did the beagle owner. If Ada adopted in 2021, so did the beagle owner. That is transfer: copy each mark from one item’s row to the other’s, in every block they share.
Transfer works in every direction. A ● in the bottom block, between a breed and a year, carries into the top blocks just the same.
Cross-checking with no ● yet
You can use the same idea before you have a match. Ada can only own the beagle or the corgi. Neither of those breeds goes with 2020. Whichever one Ada owns, Ada didn’t adopt in 2020. Cross it out.
The rule: if every item a person could still have in one category is × with something, the person is × with it too.
Worth memorizing
A ● makes two items one person.
Copy every × and ● between their rows.
Do it in every block the two items share.
No ● yet? Cross-check.
If all of a person’s options share a ×, the person gets it too.
Practice
0 of 3 rightBen owns the corgi and didn’t adopt in 2020. What can you mark in the bottom block?
Cora owns the poodle, and the poodle owner adopted in 2020. Which year did Cora adopt in?
Cora can only own the corgi or the poodle. Which year can you rule out for Cora?