Every right triangle has three sides, and only one of them has a permanent name. The hypotenuse sits straight across from the square corner and is always the longest side — it is defined by the right angle, so no acute angle gets a vote on it.

The other two names are earned, not owned. Stand at the marked angle $\theta$ (the Greek letter theta, said “thay-ta”). The side straight across the triangle from you is the opposite; the side running from you to the square corner is the adjacent. Walk over to the other acute corner and those two SWAP. Same triangle, same wood, new names — and that swap is the single most expensive mistake in this chapter, because it produces a beautifully computed answer to a question nobody asked.

Which is why the three ratios $\sin\theta = \dfrac{o}{h}$, $\cos\theta = \dfrac{a}{h}$ and $\tan\theta = \dfrac{o}{a}$ always name $\theta$ out loud. Read them as sine theta equals opposite over hypotenuse, and so on down SOH CAH TOA. The ratio is meaningless until you say whose angle it belongs to.