---
title: "CSS Flexbox"
slug: "css-flexbox"
category: "glossary"
tags: ["glossary", "frontend", "css", "layout", "flexbox", "responsive-design"]
status: "stable"
last_updated: 2026-10-01
summary: "CSS Flexbox is a one-dimensional layout model that distributes items along a main axis set by flex-direction, suited to toolbars, nav bars, and centering."
aliases: ["glossary/flex-direction"]
related: ["[[glossary/css-grid]]", "[[glossary/box-model]]", "[[frontend/css-flexbox]]", "[[frontend/css]]", "[[frontend/tailwind]]"]
---

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## Overview

`display: flex` lays a container's items along one main axis, set by `flex-direction`, and aligns them on the perpendicular cross axis. Use it for toolbars, nav links, button rows, and centering, letting content size drive the layout. When columns must line up across rows, use [[glossary/css-grid]].

## Rules

- `flex-direction` is `row` (default), `column`, or a `-reverse` variant. `justify-content` aligns along the main axis and `align-items` along the cross axis, so switching to `column` swaps what they do; that is why centering often breaks at a mobile breakpoint.
- `row` follows the writing mode's inline direction: right to left in RTL text and top to bottom in vertical `writing-mode`.
- Avoid `row-reverse` and `column-reverse` for content users read or tab through: only visual order changes, while DOM, tab, and screen-reader order do not.
- `flex-wrap: wrap` makes multi-line layouts whose lines size independently.
- Flex items default to `min-width: auto`, so long content will not shrink; set `min-width: 0` on the item to allow it.
- Use `gap` for spacing instead of margins.

```css
.navbar { display: flex; align-items: center; justify-content: space-between; gap: 1rem; }

@media (max-width: 600px) { .navbar { flex-direction: column; } }
```

## Related

- [[glossary/css-grid]]: the two-dimensional layout model.
- [[glossary/box-model]]: flex items do not collapse margins.
- [[frontend/css-flexbox]]: layout patterns in depth.
- [[frontend/css]]: CSS conventions.
- [[frontend/tailwind]]: the `flex`, `items-*`, and `justify-*` utilities.