While the function of an usu is similar to the smaller suribachi and surikogi mortars, the shape is very different, as the usu usually lacks the rough pattern in the bowl, and had a very different shaped and used pestle.
The usu is usually large sized, with a height of about 1 meter (including pedestal) and a diameter of ca. 30cm. The usu is usually made of wood or stone. The kine is a long wooden hammer with a length exceeding 1m. The usu is usually operated by two people at the same time. One person swings the kine to pound the rice in a similar motion as chopping wood, which is also an equally demanding work. Between each swing, another person puts his hand in the bowl to turn the rice. It is easy to see that the timing is critical for a safe operation of the usu.
The most common use of an usu is to pound cooked white rice into a very sticky mass in order to produce mochi. It is still frequently used in Japan in traditional restaurants and also sometimes in wealthier private households. It is said that rice pounded in an usu and kine tastes better than rice that has been processed by an electric machine.
See also: List of Japanese cooking utensils