《管子·地员》
凡是要起奏五音风调，先确立一弦而三等分之，经过四次三等分的推演以合九九八十一之数，由此产生黄钟小素的音调，便成为宫声。
凡将起五音凡首，先主一而三之，四开以合九九，以是生黄钟小素之首，以成宫。
When creating the five notes of the wind scale, first determine one string and divide it into three equal parts, and then multiply it four times to equal 9981. This produces the fundamental note of the wind scale, which becomes the note 宮.