Right-angled triangle relative to angle $\theta$:

          θ
          o
         /|
        / |
       /  | adjacent
      /   |
     /    |
    o-----o
     opposite

Trigonometric ratios link an angle to side lengths. In a right-angled triangle, the side names depend on the chosen angle $\theta$.

With $\theta$ at the top vertex and the right angle at the bottom-right corner:

• the vertical side next to $\theta$ is the adjacent side
• the sloping longest side is the hypotenuse
• the bottom horizontal side across from $\theta$ is the opposite side