The Complete Handbook of Squares

Chapter 1: Understanding Squares

A square is a measuring and marking tool that establishes or verifies a 90-degree angle—a right angle. That simple definition conceals the reality: a square is a precision instrument whose accuracy directly affects the safety, appearance, and function of everything built with it. Understanding what a square actually is, how it works, and why that precision matters is the foundation for choosing the right one, using it correctly, and recognizing when it has failed.

Before diving into types and specifications, it helps to ground squares in the real work they do. A carpenter framing a house uses a square to mark where wall studs should be cut and to verify that corners are actually square before nailing. A machinist uses a precision square to ensure that a part is positioned correctly on a lathe or mill, and any error—even a fraction of a degree—can ruin the part or create a safety hazard. A fabricator uses a square to check that welded joints are perpendicular. A woodworker uses a square to verify that a table leg is truly vertical before gluing. In each case, the square is not just a convenience; it is the reference point for everything that follows. If the square is wrong, everything built from it is wrong.

How a Square Works: The Physical Principle

A square consists of two rigid components joined at exactly 90 degrees. The simplest form—a try square—is two metal pieces (called the stock and the blade) riveted or