Every Czech beer wears a number, and almost everyone abroad reads it as strength. It isn't — it's body.
Walk into a pub anywhere in Bohemia and the taps will be labelled 10, 11, 12 — sometimes 13 if the pub is feeling generous. Ask a British or American drinker what that means and they'll usually guess alcohol. It's a reasonable guess, and it's wrong.
The number is degrees Plato: the percentage of dissolved sugar in the wort before the yeast gets to it. A 10° wort is 10 % sugar by weight. What the yeast does with that sugar afterwards determines the alcohol, and how far fermentation is pushed varies enormously between beers.
Compare two beers in our own range. Pepinova desítka is 10° and finishes at 4,1 % ABV. Postřižinský tmavý ležák is 11° and finishes at 4,3 % — barely any different despite the extra degree, because the Bavarian and caramel malts contribute sugars the yeast largely leaves alone.
So what does the degree actually tell you? Body. Fullness. How much beer there is in the beer. A 10° is designed to be drunk in quantity across an afternoon without filling you up. A 13° like Bogan has roughly a third more material in it, and you feel that on the palate long before you feel the alcohol.
This is why the Czech system is more useful than a bare ABV figure once you understand it. It tells you what kind of session you're in for, not just how it ends.
The practical rule: 10° for the afternoon, 11° with dinner, 13° when you're having one and you mean it.
