King Wen of Zhou held two-thirds of the realm, yet still served the Yin (Shang).
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From 《论语·泰伯》: 三分天下有其二，以服事殷，周之德，其可谓至德也已矣。(Holding two-thirds of the realm yet still serving the Yin — the virtue of Zhou may truly be called the highest virtue.) Confucius praises King Wen (周文王) for restraint: though powerful enough to topple the Shang, he refused to rebel against his nominal overlord.
usage
Originally about political-military dominance and restraint; today often used in business contexts for market dominance. Don't confuse with 三国 (Three Kingdoms) 'dividing the realm into three' (三分天下 as a goal).
