It really depends. For example, take lambic beer production. This is a spontaneously fermented beer from Pajottenland, near Brussels. You can easily hit 8-9%

It really depends. For example, take lambic beer production. This is a spontaneously fermented beer from Pajottenland, near Brussels. You can easily hit 8-9% alcohol. The key is that the microflora that innoculates the wort was living in the barrels. When you had a "good" barrel, you would keep brewing in it. When the barrel went "bad" you would destroy it and burn it. The entire brewery would be a source of yeast and in some breweries it was considered very bad form to clean. Even today, there are some Belgian breweries where you can get fired if you kill a spider (the spiders catch fruit flies that can bring in wort spoiling bacteria). Once you had a good, stable population of yeast/bacteria you could maintain it simply by overwhelming other beasties. When I first started homebrewing, I didn't know that much and I must have gone nearly 10 years without ever disinfecting anything. I had a definite "house flavour" in my beer, but it was far from bad. Anyway, these days, in order to get up to 10% (or higher) alcohol level, you will go with a single strain with high alcohol tolerance. But there are several traditional multi-strain cultures that do a very good job and have a very interesting flavour profile. Quite frequently you'll have one strain that ferments quickly and drops out fast with a second one that is "powdery" and hangs in suspension for months. Finally you will have a "chaining" yeast that will drag the "powdery" yeast out of suspension. You end up with having most of the flavour profile of the first yeast, while the powdery yeast will bring the terminal gravity down and the last yeast will brighten the beer. Some really famous yeast cultures (like Worthington Whiteshield) are composed of 5 or more strains working in concert. It is truly amazing what people accomplished just with selective techniques. Having said that, some styles that are currently quite high alcohol were traditionally much sweeter. Bock styles were drunk by monks during lent. These beers had to be sweet enough to sustain them, because they ate nothing else during that period. A friend of mine who did quite a lot of research in the area (because he wanted to do the same thing... :-P) reckoned that historical bocks were probably only 3-4% alcohol.