A gaiwan (Chinese 盖碗, "a cup with a lid") is a traditional Chinese cup for brewing tea, which often stays in the shadow of the Chahu teapot. This position undeservedly attracts little attention, being not any worse technically, furthermore, it has a few advantages. First of those is the speed of draining, this is especially important while brewing tea grades that can become bitter if overexposed. Second, it has no nozzle - which could be cluttered by fluffy buds. Third, the scent of aetherial oils is perceived much better on a cooling porcelain lid (this is difficult to overestimate when brewing oolongs - even those which have most moderate and weak fragrance).
The gaiwan is considered to be an invention from Sichuan province. Chahu teapots are not usually used there, tea is brewed in a gaiwan and then, drunk from it. A local legend says that it was designed by Qui Nin who was a daughter of the Governor-General of Chandu. She would put a hot cup on a wooden saucer to protect her delicate fingers from burning. This story happened during the Tan epoch. Archeological researches found out that big tea cups were already used together with a lid and a saucer in central and western regions of China during the Han dynasty. However the peculiar form of "an open flower" appeared only in the beginning of the Qin dynasty.
Traditionally, green tea grades are brewed in a gaiwan. The little split between the edge and the lid of a gaiwan is meant for sipping hot tea. That is the way how they have been doing it in Sichuan for 500 years; brewing delicate tea buds with hot water that was slightly cooled down after boiling. However, the gaiwan is used as an alternative to the usual teapot everywhere nowadays. It is especially convenient for trying something new, an unknown grade of tea.
As far as the material is concerned, thin white porcelain Jingdezhen is the best for white, green and yellow grades, as well as for slightly fermented oolongs. Gaiwans "dehua" and "ruyao" are recommended for Zhaochou Cha, Yan Cha and red grades, and ceramic gaiwans with thick walls for Lao Cha, Hei Cha and Pu'ers. Herbal tea and phyto mixtures are best of all brewed in a glass gaiwan.