# Squaring & Leveling: The 3-4-5 Rule
Three words you'll hear constantly:
- **Square** — a perfect **90°** corner.
- **Level** — perfectly **horizontal**.
- **Plumb** — perfectly **vertical**.
## The 3-4-5 rule (make a square corner)
This is the oldest trick in building. To check or create a true 90° corner:
1. From the corner, measure **3** units along one side and mark it.
2. From the same corner, measure **4** units along the other side and mark it.
3. Measure the **diagonal** between those two marks — if it's exactly **5**, the corner is **square**.
It works because **3² + 4² = 5²** (9 + 16 = 25). Use any units (feet, inches) — and for bigger, more accurate layouts use multiples like **6‑8‑10** or **9‑12‑15**.
## Checking a rectangle
A rectangle (like a wall layout or a deck) is square when its **two diagonals are equal**. Measure corner-to-corner both ways; adjust until they match.
## Level and plumb
Use a **level**: bubble centered between the lines means level (laid flat) or plumb (held vertical).
**Takeaway:** Use 3-4-5 to make a true 90° corner, equal diagonals to check a rectangle, and a level for level and plumb.
> *Educational overview — practice the hands-on skills with real tools. Repetition is how they become second nature.*
