Mirrored from Sudopedia, the Free Sudoku Reference Guide
Braid Analysis or Braiding is a solving technique that analyzes the way digits are placed in a chute. The basis for this technique is the discovery of traveling pairs. These are formed because there is only a limited number of ways 3 digits can "travel" through a chute.
Here is a diagram that shows how the 3 digits that start in the top-left intersection can be distributed in the remainder of the chute. Each rectangle represents an intersection of a line and a box, which contains 3 cells.
As you can see, there are only 4 possible distribution patterns. In 3 of these patterns, there are 2 digits that travel together, while the 3rd digit travels in the opposite direction. This is what we call a braid. One of the patterns shows that all 3 digits travel in the same direction. We call this a rope. Braids are much more frequent than ropes, but if you like to see some, take a look at the canonical grid.
It is impossible for braids and ropes to mix in a single chute. So, when we discover a singe rope in a chute, all 9 digits must follow a rope pattern.
The next diagram shows the 6 ways to place a single digit in a chute. In Braiding Analysis we call these the strands. There is always a diagonal distribution pattern, which may wrap around.
The identifiers are formed by the starting position and the traveling direction. N represents a downward direction and Z the upward direction. On the right, a merged diagram shows to which 2 strands each intersection belongs.
As we have seen, digits only have limited ways to be organized in a chute. From these limitations, we can deduce a number of rules concerning repeating pairs.
To enable us to analyze the strands, we must first map the candidates to those strands. This is a process that takes 2 steps. Here is a candidate grid that we will use as an example:
In the first step, we collect all candidates which are present in each intersection. You need to do this twice. Once for the boxrows and once for the boxcols.
boxrows - boxcols
You do not need to highlight the placed digits, but it will be useful for the next step.
In the second step, we collect all candidates which appear in all 3 intersections that belong to each strand. The result is a list of digits that each strand can contain.
floors - towers
The highlights show which digits are mandatory in their strands. You can set these when at least 2 intersections in the strand already contain this digit (using the highlights in the previous step) or when none of the 5 other strands in that floor allow this digit.
Each chute can be organized in one of 4 possible patterns.
These 4 patterns define how many digits will be assigned to N and Z strands:
There are a few rules that can help us eliminate some of these patterns:
Now we use these rules to reduce the number of possibilities in tower 2:
Although the results that you can achieve with Braid Analysis look impressive, the same result can often be achieved with less complicated techniques. You should reserve it for Sudokus which cannot be advanced any further by the solving techniques that you are familiar with.