A cornerstone is a stone set at the corner of a building's foundation, traditionally placed during a special ceremony to mark the start of construction. Builders in ancient times would carefully select this first stone because it had to be strong and precisely cut: every other stone in the building would be positioned relative to it. If the cornerstone was crooked or weak, the entire structure would be flawed.

Today, we rarely build with individual stones, but the word cornerstone has taken on a powerful metaphorical meaning. When something is described as a cornerstone, it means it's a fundamental part that everything else depends on. Trust might be the cornerstone of friendship: without it, the friendship can't stand strong. Practice is often called the cornerstone of success in sports or music because no amount of talent matters without dedicated practice supporting it.

The word carries weight and permanence. You wouldn't call something trivial a cornerstone. When the Declaration of Independence is called a cornerstone of American democracy, it means our entire system of government is built upon those founding principles. When a teacher says reading comprehension is the cornerstone of education, she means every other subject depends on that skill.

Notice how the word suggests both importance and stability: cornerstones aren't meant to shift or change. They're meant to last and hold everything else in place.