Adobe Illustrator is a vector graphics program. Your main work area for creating vector drawings is called an "artboard." It's a white square or rectangle surrounded by a gray area. If you're creating vector art and want to exactly center an object on the artboard, it can be useful to display a center mark. This appears as a cross that overlays the artboard.
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Double-click the "Artboard" tool. Its symbol is a small black outline of a square with two lighter-colored lines protruding from each corner.
Place a check in the "Show Center Mark" option under the "Display" heading.
Click the "Selection" tool to de-select the artboard. The center mark now appears in the center of the artboard.