Gong Dao Bei, literally translated as fairness cup is used in Chinese tea sessions to pour out or distribute tea into teacups.
If you have brewed a teapot /gaiwan of Chinese tea and you are to pour the tea into 4 or more teacups, you may have the issue that there may be uneven distribution of tea in terms of amount of tea and strength of the tea. You may discover some cups had more/less tea and some cups look stronger /weaker than other cups. It is especially tough to ensure an even pour out of the tea (as in the last pix) where each cup has the same colour in terms of strength of the tea. It will take lots of practice to achieve that skill. Thats where the fairness cup is useful in a tea session.
All you need to do is to empty your tea from a teapot/gaiwan into a fairness cup. This will ensure the tea when poured out from the fairness cup into tea cups are similar and 'fair' in terms of strength of the tea. Gong dao bei is an essential accessory used in tea sampling session to ensure uniform taste per infusion when sampling a tea.
I have a few fairness cups. This 1st one was given to me while I had visited a tea farm in Taiwan many years back. The 2nd pix show an old unused dark red 'wan shou hu jiang' and a rice porcelain cup. I believed their original use were as mini tea pitchers or milk jugs. I do see such similar shaped jars used to dispense milk when I am drinking tea or coffee in a restaurant. The 3rd pix are fairness cups made from glass. One is a stylized cup, with an extended glass spout, that sits in a bamboo holder while the one on its right is a double glass walled version.
One good idea is to use another Chinese teapot as a fairness cup.......or if you have a teapot with a broken lid, keep that topless teapot as a gong dao bei. In Guangzhou, I saw a new teapot being seasoned by using it as a fairness cup.
My wife is eyeing to use one of my porcelain fairness cups as a gravy cup for our year end turkey dinner.