Basic concepts of flexbox - CSS | MDN

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The flexible box layout module (usually referred to as flexbox) is a one-dimensional layout model for distributing space between items and includes numerous alignment capabilities. This article gives an outline of the main features of flexbox, which we will explore in more detail in the rest of these guides.
When we describe flexbox as being one-dimensional we are describing the fact that flexbox deals with layout in one dimension at a time — either as a row or as a column. This can be contrasted with the two-dimensional model of CSS Grid Layout, which controls columns and rows together.
When working with flexbox you need to think in terms of two axes — the main axis and the cross axis. The main axis is defined by the flex-direction property, and the cross axis runs perpendicular to it. Everything we do with flexbox refers back to these axes, so it is worth understanding how they work from the outset.
The main axis is defined by flex-direction, which has four possible values:
Should you choose row or row-reverse, your main axis will run along the row in the inline direction.
Choose column or column-reverse and your main axis will run in the block direction, from the top of the page to the bottom.
The cross axis runs perpendicular to the main axis. Therefore, if your flex-direction (main axis) is set to row or row-reverse the cross axis runs down the columns.
If your main axis is column or column-reverse then the cross axis runs along the rows.
Another vital area of understanding is how flexbox makes no assumption about the writing mode of the document. Flexbox doesn't just assume that all lines of text start at the top left of a document and run towards the right-hand side, with new lines appearing one under the other. Rather, it supports all writing modes, like other logical properties and values.
You can read more about the relationship between flexbox and writing modes in a later article; however, the following description should help explain why we do not talk about left and right and top and bottom when we describe the direction that our flex items flow in.
If the flex-direction is row and I am working in English, then the start edge of the main axis will be on the left, the end edge on the right.
If I were to work in Arabic, then the start edge of my main axis would be on the right and the end edge on the left.
In both cases the start edge of the cross-axis is at the top of the flex container and the end edge at the bottom, as both languages have a horizontal writing mode.
After a while, thinking about start and end rather than left and right becomes natural, and will be useful to you when dealing with other layout methods such as CSS Grid Layout which follow the same patterns.
An area of a document that is laid out using flexbox is called a flex container. To create a flex container, set the area's display property to flex. When we do this, the direct children of that container become flex items. You can explicitly control whether the container itself is displayed inline or in block formatting context using inline flex or inline-flex for inline flex containers or block flex or flex for block level flex containers.
As with all properties in CSS, some initial values are defined, so the contents of a new flex container will behave in the following way:
The result of this is that your items will all line up in a row, using the size of the content as their size in the main axis. If there are more items than can fit in the container, they will not wrap but will instead overflow. If some items are taller than others, all items will stretch along the full length of the cross-axis.
You can see in the live sample below how this looks. Click "Play" to open the example in the MDN Playground and edit the items or add new items to try out the initial behavior of flexbox:
Adding the flex-direction property to the flex container allows us to change the direction in which our flex items display. Setting flex-direction: row-reverse will keep the items displaying along the row, however the start and end lines are switched.
If we change flex-direction to column the main axis switches and our items now display in a column. Set column-reverse and the start and end lines are again switched.
The live sample below has flex-direction set to row-reverse. Try the other values — row, column and column-reverse — to see what happens to the content.
While flexbox is a one dimensional model, it is possible to make flex items wrap across multiple lines. If you do this, you should consider each line as a new flex container. Any space distribution will happen across each line, without reference to the previous or subsequent lines.
To cause wrapping behavior add the property flex-wrap with a value of wrap. Now, if your items are too large to all display in one line, they will wrap onto another line. The live sample below contains items that have been given a width. The total width of the items is too wide for the flex container. As flex-wrap is set to wrap, the items wrap across multiple lines. If you set it to nowrap, which is the initial value, they will shrink to fit the container. They shrink because they are using initial flexbox values, including flex-shrink: 1, that allows items to shrink. Using nowrap would cause an overflow if the items were not able to shrink, or could not shrink small enough to fit.
Find out more about wrapping flex items in the guide Mastering wrapping of flex items.
You can combine the two properties flex-direction and flex-wrap into the flex-flow shorthand.
In the live sample below, try changing the first value to one of the allowable values for flex-direction - row, row-reverse, column or column-reverse, and also change the second to wrap and nowrap.
To control the inline-size of each flex item, we target them directly via three proper