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ch is based on the width of a "0" character, em is based on the font size, which is the height of the character. for setting widths of things that contain text ch makes more sense. For padding, spacing, font sizes, inline symbols, and that kind of thing, em makes more sense.

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An em is historically based off the width of the letter M in a typeface. https://iamvdo.me/en/blog/css-font-metrics-line-height-and-v... is a fun deep dive into font sizing, which is pretty hairy.

That was my understanding as well. em was used for width and ex used for height.

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