A right angle measures 90 degrees and forms an L-shape or corner like the corner of a square or rectangle. Identifying right angles helps classify shapes (rectangles have 4 right angles; triangles may have 0 or 1).
An angle is formed wherever two lines or edges meet at a point. You already know that angles can be classified — some are wide and open, some are sharp and narrow. A right angle is the special middle case: exactly 90 degrees, the angle you get when one line stands perfectly perpendicular to another. The clearest example is the corner of a square, a piece of paper, or a floor tile. That precise L-shape is the definition. An angle that is smaller than a right angle is called acute; one that is larger is called obtuse. Being able to spot right angles specifically is the key to classifying many common shapes.
The easiest way to test whether an angle is a right angle is to hold the corner of a piece of paper (or a folded square) against it. A sheet of paper has four right angles, so any corner of it is a perfect 90-degree tester. Press one edge of the paper along one side of the angle and look at how the other side of the paper lines up with the second edge of the shape. If they line up perfectly, the angle is a right angle. If the shape's edge falls outside the paper's corner, the angle is obtuse; if it falls inside, the angle is acute. This "corner test" works on drawn shapes and on physical objects.
Right angles are what define the rectangle family of shapes. A rectangle has exactly four right angles — that is its defining property. A square is just a special rectangle where all four sides happen to be equal, so it also has four right angles. A triangle, on the other hand, has angles that must sum to 180 degrees; if one of them is 90 degrees, the other two must share the remaining 90 degrees between them, meaning they are both acute. A triangle with one right angle is called a right triangle, and that single right angle is a crucial property used in measurement and construction. Shapes without any right angles — like typical parallelograms, rhombuses, or most pentagons — have all acute or obtuse corners. Learning to spot right angles by eye, and then confirm with the corner test, gives you a fast first tool for sorting and classifying any 2D shape you encounter.
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