法華玄義釋籤 1.13

1.13

天子為萬乘之主，故云「萬乘屈膝」。言「萬乘」者，《論語》包氏引王制云：「古者井田，方里為井，十井為乘，百里之國適千乘也。諸侯之國千乘，天子之國萬乘，故云萬乘之主。

The Son of Heaven is the lord of ten thousand chariots, hence 'the lord of ten thousand chariots bowed his knees'. Regarding 'ten thousand chariots', Master Bao in the Analects cites the Royal Regulations, stating: 'In ancient times, in the well-field system, a square li was a well; ten wells made a chariot, and a state of a hundred li possessed precisely one thousand chariots.' A feudal lord's state had a thousand chariots, and the Son of Heaven's state had ten thousand chariots, hence called the lord of ten thousand chariots.