Z Index (`z-index`) is a CSS property that defines the order of overlapping HTML elements. Elements with a higher index will be placed on top of elements with a lower index.
**Note**: Z index only works on positioned elements (`position:absolute`, `position:relative`, or `position:fixed`).
#### Possible Values
```css
/* Default value if not specified */
z-index: auto;
/* Integer values */
z-index: 1;
z-index: 100;
z-index: 9999;
z-index: -1;
/* Global values */
z-index: inherit;
z-index: initial;
z-index: unset;
```
#### Example Usage
In this example, you can see three boxes displayed on top of each other in different orders using `z-index`.
<em>HTML</em>
```html
<divclass="container">
<divclass="box"id="blue"></div>
<divclass="box"id="red"></div>
<divclass="box"id="green"></div>
</div>
```
<em>CSS</em>
```css
#blue {
background-color: blue;
}
#red {
background-color: red;
}
#green {
background-color: green;
}
```
Since `z-index` wasn't defined, it will have a default value of `auto`. This is a result:
Try to change the order to Green, Blue, Red in CSS using `z-index`.
```css
#blue {
background-color: blue;
z-index: 2;
}
#red {
background-color: red;
z-index: 1;
}
#green {
background-color: green;
z-index: 3;
}
```
Your result will be:
Use Z Index if you need to put a background element below a container. You can easily place the background under every element by giving it a negative Z Index like below:
```css
#background {
z-index: -1;
}
```
#### More Information:
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