Taiji - Mill

By far the most complex and difficult puzzle mechanic to master in the game. In this area, each region with a horizontal line of the same colour must be the exact same shape - regardless if the tiles in said region are black or white.
If there is more than one coloured line in a puzzle, each shape must be unique to one another in some form. Be it having more or less tiles in it's shape, having the same number of tiles but rearranged to be different, or in rare cases occupying different spots within the same area.
For diagonal lines, the region can be rotated 90 degrees from the diagonal line tile's pivot point. These can be treated the same as their horizontal counterparts if the circumstances permit, but you'll probably have to rotate the shape anyway in most puzzles. Again, if there is more than one coloured diagonal line, you'll need to make them different from each other.
Some of the puzzles will have both horizontal and diagonal lines. Sometimes with multiple colours. If that's the case, make sure the shape is the same as their respective coloured lines.