The 500-year-old beer-drinking rule travellers must know before entering Cologne

German beer culture's regional diversity - from crisp pilsners in the north to dark, fermented lagers from Bavaria - stirs heated regional rivalry. So, which region makes the best beer of all? That depends who you ask, and where.

Today I'm in Cologne, exploring the vibrant Rhine city during a port stop on my Trafalgar river cruise.

In these parts, the king of brews is Kolsch, a pale, crisp beer that's very drinkable with a faint, fruity finish. You don't ask for "a beer" in Cologne's Brauhauser (beer halls), you order a Kolsch. Protected by law like champagne, true Kolsch must be brewed within the city limits.

At Gaffel am Dom, a typical beer hall in the shadow of Cologne's towering Gothic cathedral, a waiter approaches my table.

He's carrying a circular tray, known as a Kranz (wreath), with space for exactly 11 small glasses. Eleven, he tells me, is the Rhineland number for carnival and revelry.

The waiter explains the system, in place for more than 500 years: ice-cold Kolsch is served in a 200ml glass known as a Stange. As soon as my glass is empty, it'll be swapped for a fresh one.

Each beer costs 2.50 euros ($4); the tally will be marked on my coaster. The only way to stop this rapid-fire refreshment is to place the coaster firmly on top of my glass and admit defeat.