Lyon's most distinctive urban feature is the traboule — a passageway that cuts through a building from one street to the next, running through the courtyards of old silk merchants' houses and sometimes descending or ascending several floors. There are around 300 in Lyon, of which about 40 are regularly open to the public. They were built by the silk-weaving industry to allow bolts of fabric to be moved undercover in a city where it rains frequently.
The best concentration is in Vieux Lyon (Renaissance traboules, often with elaborate spiral staircases) and Croix-Rousse (industrial-era passages connecting the weavers' workshops). The tourist office sells a map showing the open entrances. The rule is simple: if a door is unlocked, you can walk through.