Flexbox deals with layout always in only one dimension; it's possible to specify which dimension: either vertical or horizontal.

Two axes: main and cross

Control main axis: `flex-direction: row|row-reverse|column|column-reverse`. The first two make it a horizontal layout and the latter make it a vertical layout.

Cross axis is always the opposite of `flex-direction`, so if `flex-direction: row-reverse` the cross axis is `column-reverse`.

[[TODO]] What is: "writing mode of the document"? Mentioned on the docs.
This document explains: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_flexible_box_layout/Relationship_of_flexbox_to_other_layout_methods#writing_modes