der Eckstein
cornerstone
A large, often shaped stone that is placed at the corner of a building to connect and stabilize two walls.
Der Eckstein des alten Rathauses trägt die Jahreszahl 1888.
The cornerstone of the old town hall bears the year 1888.

cornerstone
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cornerstone
A large, often shaped stone that is placed at the corner of a building to connect and stabilize two walls.
Der Eckstein des alten Rathauses trägt die Jahreszahl 1888.
The cornerstone of the old town hall bears the year 1888.

The word is made up of 'die Ecke' (the corner) and 'der Stein' (the stone). So it literally describes a stone that is placed at a corner of a building.
Der Maurer setzte den letzten Eckstein sorgfältig ein.
The mason carefully set the final cornerstone in place.
In the figurative sense, the 'Eckstein' (cornerstone) is the most important, fundamental element of something abstract. It is often used with the genitive or the preposition 'von'.
Gegenseitiger Respekt ist der Eckstein einer guten Freundschaft.
Mutual respect is the cornerstone of a good friendship.
In the figurative meaning, 'Eckstein' (cornerstone) is often connected with abstract concepts. Typical phrases are 'der Eckstein einer Beziehung' (the cornerstone of a relationship), 'eines Konzepts' (of a concept), or 'eines Unternehmens' (of a company).
Die pünktliche Lieferung ist der Eckstein unseres Geschäftsmodells.
On-time delivery is the cornerstone of our business model.
The call 'Eckstein, Eckstein, alles muss versteckt sein!' is the beginning of a very well-known counting-out rhyme. It is used by children when playing hide-and-seek.
Als die Kinder „Eckstein, Eckstein“ riefen, suchten alle schnell ein Versteck.
When the children called 'Eckstein, Eckstein', everyone quickly looked for a hiding place.
basis / base
'die Basis' and 'der Eckstein' can both refer to something fundamental for a larger whole. 'die Basis' is a neutral word for a foundation or starting point. 'der Eckstein' is more figurative and sounds stronger: it means a particularly important supporting element without which a system, idea, or organization would not be stable. In everyday language, 'Basis' is more common and simpler. 'Eckstein' fits better when you want to emphasize the central importance of a principle or element.

Respekt ist ein Eckstein unserer Zusammenarbeit.
Respect is a cornerstone of our cooperation.
Diese Daten sind die Basis für den neuen Bericht.
This data is the basis for the new report.