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In this tutorial you will learn how to easily create a diagonal pattern in Adobe Illustrator. Creating seamless patterns is a challenge in itself, but it is even harder if the pattern is supposed to be at an angle. But there is an easy trick to do so by creating a simple straight pattern first.
Create a square rectangle as a base for your pattern. You can do so by selecting the Rectangle Tool (M) from the Tools panel. Double click anywhere on your canvas. By doing so a window will pop up in which you can enter the exact size you want for your square. I chose 1 × 1 in. But you can make it as big as you want your pattern to be.
Duplicate the square, I use Command + C and Command + F. This will paste the copy of your square in exactly the same place it was before. Select one of the reference points at the bounding box in the middle and hold down Option while dragging it into the middle of the rectangle. Holding down Option will help you to keep the rectangle symmetrical from the center.
Fill this rectangle with black and change the Opacity to 20%.
Repeat the previous step with a new rectangle. This one will be filled with white.
Do it one more time and fill the new rectangle with black again. By doing these three steps you make sure that the bottom and top of your pattern repeats seamlessly. Which colors, opacities and blend modes you use is entirely up to you.
Select the three new rectangles and duplicate them. Keep the rectangles selected and turn them by 90°.
This will be our base pattern.
Open the Swatches Panel and simply drag and drop your pattern base into the panel.
Create a new bigger rectangle and fill it with your pattern. As you can see it repeats perfectly.
Select your rectangle and double click the Rotate Tool (R) from your Tools Panel.
The Rotate window will pop up. Check Preview and Uncheck Objects. By leaving only Patterns selected the rectangle will not rotate, but the pattern will. Enter an Angle of 45°.
In under 5 minutes you have created a diagonal pattern. Doing it manually is time consuming and hard to get a grip of at first. Luckily Illustrator has many, sometimes hidden, tricks to make your life easier. This is one of them. This works for every angle and so extremely easy to use for effects like scanlines, that can come at many different angles.