This architectural adjective describes the cornerstone—the foundational stone placed at the corner of a building that binds two walls together and determines alignment. Paul uses it metaphorically in Ephesians 2:20 where Christ is the ἀκρογωνιαῖος—the chief cornerstone—of God's spiritual temple, the church. Peter quotes Isaiah 28:16 in 1 Peter 2:6, describing Christ as the elect, precious ἀκρογωνιαῖον stone laid in Zion. Both apostles draw on Old Testament imagery where the cornerstone represents what is fundamental, chosen, and essential for the entire structure's integrity.