Limonene is found in the essential oils of citrus fruits and many other plant species and is the main aroma component in oranges.

The limonene molecule (C₁₀H₁₆) exists in two forms that are mirror images of each other:

l-limonene, which has a piney, turpentine smell, and

d-limonene, which has a pleasing orange scent

The different scents are the mind's interpretation of the shape of the molecules.