The 3-4-5 rule is a simple and effective method used in interior design to ensure that everything in a room is properly aligned and balanced. The rule is based on the Pythagorean theorem, which states that in a right-angled triangle, the square of the length of the hypotenuse (the longest side) is equal to the sum of the squares of the other two sides. Here’s how the 3-4-5 rule works in interior design:

1. Choose a starting point: To use the 3-4-5 rule, you need to choose a starting point that will serve as the base for all your measurements. This could be a corner of the room, a point on the wall, or any other fixed point.
2. Measure 3 units: From your starting point, measure 3 units along one wall. This could be 3 feet, 3 meters, or any other unit of measurement that works for your space.
3. Mark the point: Once you’ve measured 3 units, mark the point with a pencil or other marking tool.
4. Measure 4 units: From the same starting point, measure 4 units along the adjacent wall that forms a right angle with the first wall.
5. Mark the point: Once you’ve measured 4 units, mark the point with a pencil or other marking tool.
6. Measure 5 units: From the second point you marked, measure 5 units along the wall that is opposite the first wall you measured.
7. Check the angle: Once you’ve measured 5 units, check the angle between the first and last points you marked to ensure that it forms a right angle.

Using the 3-4-5 rule can help ensure that everything in a room is properly aligned and balanced. For example, you can use the rule to ensure that a large piece of furniture 

is properly centered on a wall, or to ensure that a rug is properly aligned with the walls of the room.

In addition to its practical applications, the 3-4-5 rule can also help create a sense of harmony and balance in a space. By ensuring that everything is properly aligned and balanced, you can create a sense of order and calm that can contribute to a relaxing and enjoyable environment.

Overall, the 3-4-5 rule is a useful tool that interior designers can use to ensure that everything in a room is properly aligned and balanced. Whether you’re designing a living room, bedroom, or any other space, the 3-4-5 rule can help you create a beautiful and harmonious environment that is both functional and aesthetically pleasing.