A lot of numbers can potentially be found when the T pattern is seen
The T pattern allows you to look at both a row and a column for potential numbers. It is found when a box has empty cells that are in the shape of a capital T.
Watch for T patterns that emerge when other numbers are solved. They can appear at be useful any time.
The T pattern is when there are four (or more) numbers in a box that line up to make a T shape with the empty cells. Here is an example puzzle with two T patterns, one in box 3 and the other in box 7. The red lines show the cells that form the Ts.
Once a T pattern is found a row and a column can be used to find numbers. In the example below several interesting things are seen.
Let's look at the T pattern in box 3. There is a 5 in row 3. This means that one of the cells in column 8 in box 3 is a 5 (see the ghost 5s in box 3). Notice that the 5 in row 1 (box 2) means that the 5 in box 3 is actually in r2c8. This isn't shown, but the ghost 5s in box 3 could have been replaced with a 5 in r2c8. This illustrates that even without the 5 in box 2 we would still know that the 5 in box 3 is in column 8.
There is a 6 in column 8 which indicates (because of the T pattern) that the 6 in box 3 must be in row 3, hence the ghost 6s in box 3.
If we now look at the T pattern in box 7 we will see ghost 2s in column 2 because of the 2 in row 7. The ghost 5s in row 7 were found because of the 5 in column 2.
The 5 that is circled in red in row 9 was found because of the ghost 5s in box 7.
You will also notice that there are 25 twins in box 6 because of the T pattern we found in box 3. See if you understand why.
These are all examples of the T pattern and how it can be used. Lots can be found in this puzzle because of the two T patterns.
As an exercise, see if you know why r2c3 in box 1 is a 4. It is not filled in but it is a 4.
The T pattern can usually be found in medium to hard puzzles, sometimes in expert level puzzles. They can be useful in any skill level puzzle.
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