noun which may take the genitive case particle 'no'
keystone; cornerstone; important stone in a construction
Refers to a structurally or symbolically important stone in a building or stone structure, such as a keystone in an arch or a cornerstone in a foundation.
アーチの親石が全体を支えている。
The keystone of the arch supports the whole structure.
この城壁の親石は特に大きくて重い。
The cornerstone of this castle wall is especially large and heavy.
要石 also means keystone or pivotal stone, but is more commonly used metaphorically for a central or crucial element in any system, while 親石 is more literal and architectural.
Compound of 親 (oya, 'parent' or 'main') and 石 (ishi, 'stone'), literally 'parent stone'. The exact historical derivation is uncertain, but the term is used in traditional Japanese architecture and stonework.