Gotoubun Principles

If one of you makes a mistake, then overcome it together. If one of you is happy, share it with the others. Joy, sorrow, anger, love...are shared among the five of us...in five equal parts! The "Gotoubun Principles" is first mentioned: the Nakano Quintuplets are supposed to share every emotions (joy, sadness, anger, love) with each other and split it equally.

Origin
The Gotoubun Principles are passed down from Nakano Quintuplets Grandfather.

Appearance/Mention

 * Chapter 12: The sisters describe the Gotoubun Principles for the first time.
 * Chapter 50: The sisters give up their old apartment so they can be with Fuutarou. They will be alright as long as the five of them are together.
 * Chapter 56: Despite only Yotsuba failing, the other sisters willingly transfer schools with her. This is due to the value of "being together as five" from their mother.
 * Extra Bonus Comic of Volume 5: Due to the game bought by Yotsuba not allowing 5 players, Nino suggests buying 4 more consoles so they can play together.

Relevance to the Story
The principles is shown to stabilize and keep the relationship among the quints in check.