Plural of cornerstone; the fundamental or most important elements that something is built upon, or the first stones laid in a building's corner.
From 'corner' plus 'stone,' originally referring to the essential stone placed at the corner angle of a building's foundation. Used metaphorically since medieval times for fundamental principles.
The saying 'laying cornerstones' comes from when builders would literally place the first stone at a building's corner—now it means creating the foundation for anything important!
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