For artwork with repetition such as patterns or stylized effects e.g. dots, Symbols are a handy feature of Adobe Illustrator.
The Symbols palette with 3 custom symbols.
What is a Symbol?
Symbols can be any object created in Illustrator, e.g. the red squares and red circle above.
Once placed on the artboard/document a symbol is called a symbol instance or simply an instance.
Important Note
The most important thing regarding using symbols in artwork that you later want to replace with a different symbol is: you must create your initial design with symbols. You can only replace one symbol with another symbol.
Creating Symbols
1. Create your object i.e. a square.
2. Select your object.
3. Click the New Symbol icon on the Symbols palette. See below.
The object is now a symbol in the Symbols palette.
4. Delete the object.
5. Click and drag the symbol onto the artboard/document.
6. Duplicate the symbol as needed and arrange your artwork.
Replacing Symbols
1. Create the new object that will replace the multiple symbols in your artwork.
2. Create a new symbol from the object and delete the object.
3. Click the new symbol in the Symbols palette and hold down the Alt key, drag it onto the symbol (also in the Symbols palette) you want to replace.
4. All the symbol instances in your artwork will change to the new symbol.
Example
I replaced the red square symbol with a smaller blue circle.
Notice that the relative positions are the same after the change even though the circles are smaller. The grey lines are just to show positioning of instances.
This is an advantage of using symbols. If you wanted to replace square objects with smaller circles and retain the relative positions you would have to do it manually; one by one.
