要かなめ石いし

noun

keystone; linchpin; cornerstone

Refers to the central stone at the top of an arch that locks the others in place; also used figuratively for the most important part of something.

• アーチの要かなめ石いしがなければ全ぜん体たいが崩くずれてしまう。

  Without the keystone of the arch, the whole thing would collapse.

• 彼かれはチームの要かなめ石いしだ。

  He is the linchpin of the team.