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.
Watch this video for more info and examples of the T pattern
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