Every triangle in this chapter has one square corner — the right angle — and the side straight across from it is the hypotenuse (say it with confidence: hy-POT-en-use). It is always the longest of the three sides, no exceptions, ever. Spot the square corner, look across the triangle, and you’ve found it.

The other two sides earn their names from whichever angle you care about. Exams mark one corner with the Greek letter $\theta$ (theta — say THAY-ta). The side straight across from $\theta$ is the opposite; the side that runs from $\theta$ to the square corner is the adjacent — it just means next to. The hypotenuse keeps its name no matter what.