Chuk

Chuk is a wooden box painted blue and the bottom is narrower than the top.
It was introduced from China during the reign of King Yejong of Goryeo dynasty and manly used in music at confucian shrines.
There is a hole on the top of the rectangular body, and a wooden hammer is used to strike down upon the hole to produce a resounding thump.
The chuk signats the beginning of a performance, and it is always placed on the east side of any musical ensemble.