The ratio between the lengths of the opposite side and the hypotenuse in a right triangle for a specific angle, $\theta ,$ is called the sine of $\theta$ and is written $\sin (\theta ).$

The ratio is always the same for any given angle. If the hypotenuse is, for example, twice as long as the opposite side, the ratio is $\sin (\theta )=\frac{1}{2}.$ This quotient holds true because the triangles are bound by similarity. Sine only states the ratio between the opposite side and the hypotenuse, it gives no indication about the lengths of the individual sides.