ic
The width of the water ideograph — the invisible grid of the Japanese page.
One ic is the horizontal advance of the ideograph 水 (water, U+6C34), the character that serves as the width standard in CJK typography — Chinese, Japanese and Korean, where glyphs occupy equal squares. With no way to measure it, it is worth 1em.
It is the East's ch: where a Latin line is measured in zeros, a Japanese line is measured in ideographs. max-width: 20ic says twenty characters per line — the exact count Japanese composition standards ask for.
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p {
max-width: 10ic;
}