The cha hai (茶海), also known as gong dao bei (公道杯), is a cornerstone in the preparation of Chinese teas and tea ceremonies.
In China it is common practice to serve tea from small teapots and drink from small cups. While it is practical to pour directly from the tea pot, the small portion of tea in each cup will have a different flavour. The richness of the tea alternates between cups because the leaves freely float and continue to steep in the water while pouring. As a result, the first cup may taste mild in comparison to the intensely rich last cup.
The cha hai decanter offers a solution to this problem. By using the decanter as an intermediary, the tea in all its degrees of intensity blends within the decanter and thus each cup poured from the decanter will have the same richness. The second name for the decanter, gong dao bei, is a reference to this functionality and can be translated as "fairness cup".