The moktak is the instrument a visitor to a Korean temple hears first and remembers longest: a dry, hollow, slightly hollow-bellied knock, struck at walking pace under chanting. Its job is metrical. Korean liturgical chanting is long, communal and largely unaccompanied, and the wooden fish holds a room of voices together through recitation that can run for an hour or more. A practised chanter varies the stroke to signal a change of section, an acceleration towards the end of a passage, or the moment a congregation should bow, so the instrument is also a set of instructions that need no words.
Physically it is a block of hardwood, hollowed through a slit that runs along the underside, with the two lips of the slit acting as the fish's mouth. The wood is chosen and seasoned for tone, and the walls are thinned until the block rings rather than thuds. Handheld instruments are small enough to carry in one hand with the beater in the other. The same principle scaled up produces the mogeo, the long hanging fish carved with a dragon's head that lives in the temple's bell pavilion, hollowed deeply enough that a person can strike the inside of it with a two handled club. The beater is a separate object with its own shaping, weighted at the head and balanced so that a chanter can strike for an hour without tiring, and a worn instrument is repaired and re-thinned rather than replaced.