1/ When flex-basis is 0, items are themselves sized proportionally according to their flex ratios. An item with a flex ratio of 2 would be twice as wide as one with a flex ratio of 1.

flex container

absolute flex

flex direction

relative flex

2/ The edges of the content area.

Content edge

Outer edge

Inner edge

Area edge

3/ A system in which every element in a document generates a rectangular box to which properties such as width, height, padding, borders, and margins can be applied.

Element model

Box model

Rectangular model

Grid model

4/ The direction in which flex items are laid out as well as whether they are permitted to wrap onto additional lines.

flex wrap

flow

flex lines

flex direction