There are two main ways of preparing matcha for tea ceremony: thick (濃茶, koicha?) and thin (薄茶, usucha?), with the best quality tea leaves used in preparing thick tea. Historically, the tea leaves used as packing material for the koicha leaves in the tea urn (茶壺, chatsubo?) would be served as thin tea. Japanese historical documents about tea ceremony that differentiate between usucha and koicha first appear in the Tenmon era (1532–55). The first documented appearance of the term koicha is in 1575.
As the terms imply, koicha is a thick blend of matcha and hot water that requires about three times as much tea to the equivalent amount of water than usucha. To prepare usucha, matcha and hot water are whipped using the tea whisk (茶筅, chasen?), while koicha is kneaded with the whisk to smoothly blend the large amount of powdered tea with the water.
Thin tea is served to each guest in an individual bowl, while one bowl of thick tea is shared among several guests. This style of sharing a bowl of koicha first appeared in historical documents in 1586, and is a method considered to have been invented by Sen no Rikyū.
The most important part of a chaji is the preparation and drinking of koicha, which is followed by usucha. A chakai may involve only the preparation and serving of thin tea (and accompanying confections), representing the more relaxed, finishing portion of a chaji.
Read more about this topic: Japanese Tea Ceremony