Fun fact, the Altbier isn’t (purely) an ale or a lager. As you may know the two ‘families’ or ‘types’ of beers are ales and lagers. All styles of beers can be classified as either ale or lager.

For example stouts, saisons, and IPAs are ales.
Commons, pilsners, and bocks are lagers.

So what makes a beer an ale vs a lager?

The short answer is to do with the yeast and how it’s fermented. Ales are fermented with top-fermenting yeast (which floats and accumulates at the top of the tank) at warm temperatures (60˚–70˚F), whereas lagers are fermented with bottom-fermenting yeast (which sinks and packs at the bottom of the tank) at cold temperatures (35˚–50˚F).

Because of their warm fermentations, ales can generally ferment and age in a relatively short period of time (3-5 weeks). On the other hand, lagers take 1-2 weeks to ferment out before they lager (lager is a verb, too!) for 5-7 weeks.

So those are “the rules”, cool?
Great time to break them.

In the case of Altbier, it’s brewed with a top-fermented yeast (like ales) BUT fermented at a slightly cooler temperature (like lagers) making it a hybrid AND also the beer is lagered (yes, lager is a noun AND a verb)

What is lagering?

Lagering is a form of beer maturation on the yeast that usually lasts for several weeks, if not months, at or near-freezing temperatures, after fermentation and before filtration and/or packaging of the beer. Lagering is employed ALMOST exclusively for bottom-fermented beer styles—hence the name “lager” for these beers—and only rarely for top-fermented beer styles, that is, ales.

Why do brewers lager beer?

During lagering, beer undergoes subtle, but significant, flavour-altering biochemical processes that are responsible for the crisp and clean taste we usually associate with lager beers. Lagering reduces any acetic and lactic acids, for instance, to fruity-tasting esters. Likewise, any residual acetaldehyde (which can contribute a raw green apple flavour to beer) can decrease by as much as 20% to 70%; and the volatile, butterscotch-tasting diacetyl, as well as their precursors, are reduced by as much as two-thirds.

Why do brewers lager some ales?

By mixing ale yeast with techniques most commonly used for lagers, brewers are able to borrow some characteristics of each classification of beer. In the case of Altbier, we get chocolate and coffee notes (like in your favourite stout) but a light (or thin) body (like a good ol’ pilsner). Brewers are artists and have been pushing the boundaries of their craft ever since it was invented.