This also shows how to use the “locked”
squares. In the column on the far left there are three missing
squares. The numbers are 4, 5, and 9. The 9 and 4 are in the same
squares so those two squares
are “locked” up in that they are the only two numbers that can go
there. Thus the 5 has to go in the blue square.
But also the 5 has to go in the green squares
in the UL box and the 5 has to go in the red squares of the LL box.
Therefore the 5 in the ML box has to be in the far left column in
the blue square. The green and red squares have the 5 locked up in
those columns. Even though we do not know which square they are in,
we know they are in those two columns and have those locked.
Another example:
The ones are in the red squares because of the
1s that are above and to the left.
That means the
1 is in the green square.
If you look at the middle three boxes,
the number one has to be in
one of the blue squares. That means they have the outer two columns
locked up and the one
must be in the middle column of the MM box.
Because of the two 1s
beside it, the number one is
then placed in the yellow box.
So we found out it was there even
though we are not certain
which blue square the ones are located in above and below it. We
also needed to know that the
1s were in the two red boxes.
