This is something you may hear regarding the differences between tequila and mezcal. Previously this little dicho (or saying) did bear some relevance as both tequila and mezcals are made from the maguey (or agave). The main difference was that mezcal could be made from any number of agaves (1) whilst tequila could be made from only one variety (2).

1. more than 40 species of agave are listed as mezcal agaves. Most commonly used is espadín “smallsword” (Agave angustifolia (Haw.), var. espadín). The next most important are arroqueño (Agave americana (L.) var. oaxacensis, sub-variety arroqueño),cirial (Agave karwinskii (Zucc.), barril (Agave rodacantha (Zucc.) var. barril), mexicano (Agave macroacantha or Agave rhodacantha var. mexicano, also called dobadaan) and cincoañero (Agave canatala Roxb). The most famous wild agave is tobalá (Agave potatorum (Zucc.)Others include madrecuixe, tepeztate, and jabalí. Various other varieties of Agave karwinskii are also used, such as bicuixe and madrecuixe
2. Agave tequilana F.A.C.Weber (the “blue” agave)

Tequila is protected by both a NOM (1) and a DOT (2). These both reflect how, where and from what tequila can be produced (3).

1. The NOM is the Norma Oficial Mexicana or, in English, the Normative Number.  It is a seal guaranteeing that this tequila or mezcal is made to government standards. The number after NOM is the distillery registration number as assigned by the government. NOM does not indicate the location of the distillery, merely the parent company or – in the case where a company leases space in a plant – the physical plant where the tequila was manufactured.
2. The DOT is the Denomination of Origin Tequila – is the name of a geographical region or specific area that is recognized by official rules to produce certain foods with special characteristics related to location. Some of these include champagne, serrano ham, parmigiana reggiano (cheese), stilton (cheese), yong feng chilli sauce (China), Ibérico ham. There are many more.
3. See Post : Tequila and Mezcal : A Primer : How to read a tequila or mezcal label for further information on NOM and DOT conditions.

Well, Breaking News people….We can no longer say All Tequila is Mezcal BUT not all Mezcal is Tequila. This is because now mezcal is protected by its own denomination of origin. Mezcal now has rules (1) about where and from what it can be produced to be able to be called Mezcal.

1. well since 1995 anyway but these rules are in a state of flux and are quite problematic. There are accusations of bribery and corruption relating to areas not normally known for mezcal production that have in essence bought the denomination.

Now I believe it is more correct to say that “All tequila (and mezcal) are agave spirits but not all agave spirits are tequila (or mezcal)”