《黄帝内经·素问·六节藏象论》
六六之节是指用干支配合计数的六十天轮回六次为一年，用来确定天道的尺度；九九制会是指用长度为九寸的黄钟律管标定的乘九的长度测量天道的准度，以应和万物气化的度数。
夫六六之节，九九制会者，所以正天之度，气之数也。
The Six Six Partition refers to the sixty-day cycle being used in conjunction with the Trigrams and Stems, thereby adding up six times, six times in turn produces a year. This is the yardstick used to quantify the ways of Heaven. The Nine Nine Standard refers to a pitch pipe that is nine inches in length standardising the measurements of multiples of nine that are used to measure the precision of the ways of Heaven, so as to maintain the proper relationships between the changes of qi and material forms.