The meridian clock zǐ wǔ liú zhù · 子午流注

A classical scheme in which each of the twelve primary meridians is assigned a two-hour period of the day, with Qi said to be most active in that meridian at that time.

It is used as a memory aid and as a framework for some classical timing rules. It should not be read as a claims about measurable physiology; the value is organisational.