Capstone, keystone or cornerstone

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The terms capstone, keystone and cornerstone can be confusing. We will examine the difference between the definitions of capstone, keystone and cornerstone, where these words came from and some examples of their use in sentences.

A capstone is a finishing stone atop an exterior wall or roof or other exterior architectural feature. The capstone protects the masonry, causing water to flow in a certain way as to mitigate erosion. The term capstone is a closed compound word, which is a word that is made up of two words joined together without hyphens or spaces. This type of compound is also called a solid compound word. The word capstone has been in use since the 1680s. The word capstone may used figuratively, but the most common use is literal.

A keystone is the central stone placed at the top of an arch. The keystone is the apex of an arch, without it the arch would not stand. The keystone is placed last when constructing an arch, locking all the other stones into place. It allows the arch to bear its own weight. The word keystone is often used figuratively to mean the central idea of a philosophy, process, business proposition or principle upon which the entire philosophy, process, business proposition or principle stands. Pennsylvania is known as the Keystone State, due to its geographic position in the original constellation of American colonies. The term keystone has been in use since mid-1600s. Keystone is also a closed compound word. The keystone is sometimes referred to as the capstone, but strictly speaking, the two terms have different definitions.

A cornerstone is the first stone laid when constructing a masonry foundation. It is considered the most important stone in the building, as all other stones are laid in reference to this first, cornerstone. In times past, offerings might be made to the gods and interred under the cornerstone. Today, many large buildings have ceremonial cornerstones that are engraved with those who sponsored the building of the structure, a motto of the institution building the structure, etc. Some cornerstones include time capsules. These ceremonial cornerstones are generally not true cornerstones. The word cornerstone is used figuratively to mean an idea or feature upon which a theory, philosophy, process or system is based. The word cornerstone is also a closed compound word, and has been in used in its literal sense since the 1200s.


If not for the research and advocacy of the historical association, small existing details from the old yard, like the pointed capstones along the perimeter wall of Nickerson Field, might go unnoticed. (The New York Times)

Graduate students in the Doctor of Physical Therapy Program at Misericordia University presented their capstone research projects at the fifth Annual Student Research Symposium: Current Research in Physical Therapy. (The Abington Journal)

“The magnitude of the ultimate decline will largely reflect the consensus conclusion surrounding the ongoing trade dispute, which represents the keystone in this arch of agita,” the firm said. (Forbes Magazine)

In 1965, workers installed the final piece of St. Louis’ Gateway Arch, a 10-ton keystone. (UPI)

When they pulled it out, they found a box inside the concrete cornerstone that for decades was embedded in the brick by the entrance of the library with “1968” printed on it. (The Ithaca Voice)

Attorney General-elect Jason Ravnsborg told The Associated Press recently that ending presumptive probation would be the cornerstone of his legislative agenda for the upcoming session, saying the move would give the courts more flexibility. (The Rapid City Journal)