The rules that define tequila

Tequila is a Denomination of Origin product. Like Champagne or Cognac, the name is legally protected, and only spirit made under specific conditions can use it.

To be sold as tequila, the spirit must:

• Come from the denomination of origin: all of Jalisco, plus designated municipalities in Nayarit, Tamaulipas, Michoacán and Guanajuato.
• Be made from blue Weber agave (Agave tequilana Weber, variedad azul). Plain "tequila" — known as mixto — needs at least 51% of its sugars from agave; the rest can come from other sugars. Bottles labeled 100% de agave must use blue agave only.
• Be produced under the Consejo Regulador del Tequila (CRT), which certifies and audits every distillery and assigns each one a NOM number.
• Be bottled in Mexico if it's 100% de agave. Mixto can be shipped in bulk and bottled abroad.
• Be between 35% and 55% alcohol by volume.

What the rules don't cover is much of what enthusiasts care about. Additives are allowed up to 1% without disclosure. Diffusers, autoclaves and column stills are all permitted. That gap between the legal minimum and traditional practice is exactly where programs like additive-free verification — and sites like this one — come in.