curriculum/challenges/english/blocks/top-introduction-to-flexbox/6571c34668e4b3b17d3957f9.md
No matter which direction you're using, you need to think of your flex-containers as having 2 axes: the main axis and the cross axis. It is the direction of these axes that changes when the flex-direction is changed. In most circumstances, flex-direction: row puts the main axis horizontal (left-to-right), and column puts the main axis vertical (top-to-bottom).
In other words, in our very first example, you put display: flex on a div and it arranged its children horizontally. This is a demonstration of flex-direction: row, the default setting. The following example is very similar. If you uncomment the line that says flex-direction: column, those divs will stack vertically.
Which CSS property changes the main axis direction in flex containers to arrange items from top to bottom?
align-items
flex-flow
flex-direction
justify-content
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