Why do bottles of liquor list both alcohol % and “proof”? Also, why only hard liquor and not drinks with lower alcohol content (beer/wine etc.)?

Question

Isn’t proof just double the percentage?

For example, a scotch with 40% alcohol is “80 Proof”

Why list both and why only on hard liquor?

In: 20

Answer 1

They aren’t – it’s that the US retained traditional measurements for everything, including alcohol so the labeling reflects that. If the products are not labeled for sale in the US/North America they normally only list % alcohol.