The Lune is a very simple form modeled after the haiku.
The Lune is also known as the American Haiku.
It was first created by the poet Robert Kelly (truly a great poet) and was a result of Kelly's frustration with English haiku. After much experimentation, he settled on a 13-syllable, self-contained poem that has 5 syllables in the first line, 3 syllables in the second line and 5 syllable in the final line.
Unlike haiku, there are no other rules. No need for a cutting word. Rhymes are fine; subject matter is open. While there are less syllables to use, this form has a little more freedom.
There is a variant Lune created by poet Jack Collom. His form is also a self-contained tercet, but his poem is word-based (not syllable-based) and has the structure of 3 words in the first line, 5 words in the second line and 3 words in the final line.
As with Kelly's lune, there are no other rules.