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

Yes proof is 2x the percentage of ethonol in the liquid.

As for why only hard spirits are measured with proof. It’s a history holdover.