Pattern Design for the Rest of Us

Popular Pattern Types: Plaids

One simple way to create plaids is to start with a source image that has uniform bands of colors (stripes). Then rotate the image so that the stripes are oriented diagonally (at 45°) and apply the Quarter-turns & rotated mirrors symmetry , with a strong (close to 100%) blending.
Sample plaid pattern
You can create stripes in SymmetryMill and export them as a pattern. Then load the image back in SymmetryMill and use it as a source for your plaids.
Reducing the number of colors in the source stripes (for example, in Photoshop) often produces cleaner traditional plaids.