This is a Korean Form consisting of three sections, a line each,
3,4,3,4
3,4,3,4,
3,6,3,4
With out any rhyme pattern.
Westernized it has become
3,4,
3,4
3,4,
3,4,
3,6,
3,4
This can be used to accent the differences of the lines,
Where the final line ( couplet ) is the twist of the poem.
Evolution to the Western Form tends to discount this
In favor of a rhyme pattern that ties the line together.