Taken from this informative Sado Nyumon.

Chasen (tea whisk) is a tool made of bamboo and used for whisking together matcha and hot water poured into a tea bowl.

A thin thread is plaited through to seperate (the tines) half way up the approx. 10 cm long bamboo tube, and there are various versions according to school and purpose. Omotesenke uses susutake, Urasenke in the beginning of the school mostly uses shirotake (hachiku), and Mushanokojisenke uses shichiku (kurotake).

The shape of the tip varies according to the school. In Mushanokojisenke, the tip of the chasen is straight, and the outer tines are curved inward. In the Urasenke school, at first the tip was curved, but it seems it departed from Rikyu’s style. Kankyuuan school (Mushanokojisenke) has the closest shape to the Rikyu’s style.

Of whisk shapes, the soft “kazuho” is used for thin tea and the “araho” with a firmer and tighter tip and about half as many tines is used for thick tea. There is also a tenmoku chasen used with a tenmoku chawan (Chinese tea bowl) and a naga chasen (long tea whisk) used with a tsutsu chawan (cyclindical).

The word chasen first appeared in “A Treatise on a General Survey of Tea” by the Emperor Huizong of the Song Dynasty.

A whisk, or a tea whisk (chasen) is made from aged bamboo. The best have a thick, heavy base with a light, strong top. The strength of the whisk’s base and top must not be little. Also it should have the shape of ridges on a sword. When the base is thick and heavy it is easy to apply power when using it. When the top is light and strong with sword ridges, even over beating it, the floating foam is not produced.

But during the Ming Dynasty of the 15th century, matcha declined and the chasen was lost. In the “Graphic Tribute to Tea Utensils” of the Southern Song Dynasty (1269), an image of a chasen is included in “The Director of Bamboo”. The whisk shape is long and doesn’t divide the inner and outer tines. The tool used with a type of tea porriage called “furicha (okecha)” remaining in various places such as the Bote Tea in Aichi Prefecture, Botebote Tea in Shimane Prefecture, Batabata Tea in Toyama Prefecture, Bukubuku Tea in Okinawa Prefecture and Fi Tea in Kagoshima Prefecture bears a resemblance.

It is said that the modern chasen with seperated inner and outer tines was developed by Takayama Souzei (1455), a retainer of the lord Yamana Danjou and the teacher of the Kitano Renga Society, at the request of Murata Jukou who was cheif preist of the nearby Shoumyou-ji Temple.