---
title: Understanding CSS z-index
slug: Web/CSS/CSS_Positioning/Understanding_z_index
tags:
  - Advanced
  - CSS
  - Guide
  - NeedsTranslation
  - Reference
  - TopicStub
  - Understanding_CSS_z-index
  - z-index
translation_of: Web/CSS/CSS_Positioning/Understanding_z_index
---
<div>{{cssref}}</div>

<p>In the most basic cases, <a href="/en-US/docs/Web/HTML">HTML</a> pages can be considered two-dimensional, because text, images, and other elements are arranged on the page without overlapping. In this case, there is a single rendering flow, and all elements are aware of the space taken by others. <span class="seoSummary">The {{cssxref("z-index")}} attribute lets you adjust the order of the layering of objects when rendering content.</span></p>

<blockquote>
<p><em>In CSS 2.1, each box has a position in three dimensions. In addition to their horizontal and vertical positions, boxes lie along a "z-axis" and are formatted one on top of the other. Z-axis positions are particularly relevant when boxes overlap visually.</em></p>
</blockquote>

<p>(from <a class="external" href="http://www.w3.org/TR/CSS21/visuren.html#z-index">CSS 2.1 Section 9.9.1 - Layered presentation</a>)</p>

<p>This means that CSS style rules allow you to position boxes on layers in addition to the normal rendering layer (layer 0). The Z position of each layer is expressed as an integer representing the stacking order for rendering. Greater numbers mean closer to the observer. Z position can be controlled with the CSS <code>z-index</code> property.</p>

<p>Using <code>z-index</code> appears extremely easy: a single property, assigned a single integer number, with an easy-to-understand behaviour. However, when <code>z-index</code> is applied to complex hierarchies of HTML elements, its behaviour can be hard to understand or predict. This is due to complex stacking rules. In fact a dedicated section has been reserved in the CSS specification <a class="external" href="http://www.w3.org/TR/CSS21/zindex.html">CSS-2.1 Appendix E</a> to explain these rules better.</p>

<p>This article will try to explain those rules, with some simplification and several examples.</p>

<ol>
 <li><a href="/en/CSS/Understanding_z-index/Stacking_without_z-index">Stacking without the z-index property</a>: The stacking rules that apply when <code>z-index</code> is not used.</li>
 <li><a href="/en/CSS/Understanding_z-index/Stacking_and_float">Stacking with floated blocks</a>: How floating elements are handled with stacking.</li>
 <li><a href="/en/CSS/Understanding_z-index/Adding_z-index">Using z-index</a>: How to use <code>z-index</code> to change default stacking.</li>
 <li><a href="/en/CSS/Understanding_z-index/The_stacking_context">The stacking context</a>: Notes on the stacking context.</li>
 <li><a href="/en/CSS/Understanding_z-index/Stacking_context_example_1">Stacking context example 1</a>: 2-level HTML hierarchy, z-index on the last level</li>
 <li><a href="/en/CSS/Understanding_z-index/Stacking_context_example_2">Stacking context example 2</a>: 2-level HTML hierarchy, z-index on all levels</li>
 <li><a href="/en/CSS/Understanding_z-index/Stacking_context_example_3">Stacking context example 3</a>: 3-level HTML hierarchy, z-index on the second level</li>
</ol>

<div class="originaldocinfo">
<h2 id="Original_Document_Information" name="Original_Document_Information">Original Document Information</h2>

<ul>
 <li>Author(s): Paolo Lombardi</li>
 <li>This article is the English translation of an article I wrote in Italian for <a class="external" href="http://www.yappy.it">YappY</a>. I grant the right to share all the content under the <a class="external" href="http://creativecommons.org/licenses/by-sa/2.0/">Creative Commons: Attribution-Sharealike license</a>.</li>
 <li>Last Updated Date: July 9, 2005</li>
</ul>

<p><small><em>Author's note: Thanks to Wladimir Palant and Rod Whiteley for the review.</em></small></p>
</div>