foundation stone

noun2

1. •
   A stone laid during the construction of a (usually important) building, the laying of which is usually marked by a ceremony.

   • He laid the foundation stone on August 1 1847, and then set around 2,000 workmen loose on the undertaking. The station opened exactly one year later on August 1 1848. — 2021
2. •
   (figurative) The basis of something.