Why a Paper Plane works with four equal parts, and how to bend them

The Paper Plane is a modern classic with a party trick: bourbon, Amaro Nonino, Aperol and lemon juice in equal parts. No fiddly ratios to remember, and each ingredient does one clear job.

Sam Ross built it as a riff on the Last Word in the late 2000s. Here is why equal parts balance, and a slider to bend the rule and taste what changes.