Nesting flex containers is a powerful technique for building layouts that go beyond simple rows or columns. You use nested flex containers when you need different groups of items to have their own independent flex behaviors inside a larger flex layout. In flexbox, any element with display: flex becomes a flex container for its direct children, and those children become flex items. If one of those children is itself set to display: flex, it acts as both a flex item (to its parent) and a flex container (to its own children). This parent-child relationship allows you to create flexible, multi-dimensional layouts without leaving the flexbox model. You reach for nesting when you want, for example, columns within a row, or when you need sub-sections of your layout to align or wrap differently from the main container.

Nesting flex containers like this lets you organize content in a grid-like pattern, even though flexbox itself is not a grid system. In the example above, the .outer container arranges its two .inner children side by side in a row, thanks to display: flex on .outer. Each .inner container is also a flex container, but with flex-direction: column, so its children stack vertically. This combination produces two columns, each with two stacked boxes—a structure similar to a 2x2 grid. By nesting flex containers, you can mix rows and columns as needed, giving you fine control over your layout. This approach is especially useful for building complex UI structures like dashboards, product grids, or any interface where you want both horizontal and vertical grouping, but still want the flexibility and alignment features of flexbox.