#Pentagon September 11

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September 2026 several of us are going to take on the 30-Day Architectural Coach4aday Challenge each day, we will define an architectural term and provide examples of where it is located. This challenge will let observation, travel, history, and photography all intersect. For Day 11 it is the PENTAGON.

Today’s post is an intersection of geometry, architecture, and history. In geometry, a pentagon is a five-sided polygon with five angles. In architecture, the Pentagon is one of the world’s most recognizable buildings and serves as the headquarters of the U.S. Department of Defense. And historically, today marks the 25th anniversary of the 9/11 attacks on the United States. One of the four hijacked planes crashed into the Pentagon.

There is an aerial photo of the Pentagon in DC at this link.

30-Day Architectural Challenge Guidelines

If you have ever watched Seinfeld, you know that George Constanza always wanted to pretend to be an architect

Well, this month’s challenge let’s all of us that are not architects learn a little more about their language. To do that we have a simple challenge.

Every day we’ll learn one architectural term. Your challenge is to find an example—preferably one you’ve actually seen—and share a photograph, location and brief explanation.

Day-11 Pentagon

It is the 2nd largest office building in the world. Only ranked behind Surat Diamond Bourse.

What makes the Pentagon distinctive it’s use of the number 5

• Five-sided shape: The building is a regular pentagon, with five equal sides and five corresponding interior sections.
• Five concentric rings: Inside the building are five separate rings, labeled A through E. E Ring is the outermost and A Ring is the innermost.
• Central courtyard: At the center is a large five-acre open courtyard, creating a dramatic void within the enormous structure.
• Five-story height: The main building rises five above-ground floors, reinforcing the five-sided design.
• Exceptional efficiency: Despite its enormous size, the five-sided configuration creates relatively short distances between different parts of the building. Its compact design was particularly valuable for a headquarters requiring extensive communication and movement.