From Wikipedia:

Yonkoma manga (4コマ漫画, “four cell manga” or 4-koma for short), a comic strip format, generally consists of gag comic strips within four panels of equal size ordered from top to bottom.

And also:

Traditionally, yonkoma follow a structure known as kishōtenketsu. This word is a compound formed from the following Japanese kanji characters:

• • Ki (起): The first panel forms the basis of the story; it sets the scene.
  • Shō (承): The second panel develops upon the foundation of the story laid down in the first panel.
  • Ten (転): The third panel is the climax, in which an unforeseen development occurs.
  • Ketsu (結): The fourth panel is the conclusion, in which the effects of the third panel are seen.

Further Reading:

https://en.wikipedia.org/wiki/Yonkoma

https://en.wikipedia.org/wiki/Kish%C5%8Dtenketsu

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