cqi

1cqi is 1% of the container’s inline size: the width in horizontal text and the height in vertical text. It is the unit most components want, because container-type: inline-size leaves the container’s height free to follow its content, and that is enough for anything that adapts to available width.

Which axis counts depends on the writing mode of the element the unit is used on, not on the container’s. In Chromium, a child with writing-mode: horizontal-tb inside a vertical container measured cqi along the container’s width, while a vertical sibling measured along its height. The specification, read for this note, says only that each unit is evaluated on “the relevant axis described by the unit” and does not say whose writing mode decides it, so this is what Chromium does and not something the text promises. Only Chromium was tested. In a page of one writing mode the question does not come up. It matters for CJK layouts that mix directions.

Write cqi in place of cqw when the component could appear in either direction, and keep cqw when you mean the physical width. For one-language horizontal sites the two give the same number: 20cqi and 20cqw were both 80px in a container 400px wide.