The free tapa
The word tapa means a lid. The standard story of the origin is that a drinker's glass of sherry was covered with a slice of bread or ham to keep out flies, and that the cover eventually became the snack. It is the same logic as the beer mat. Another tale credits a medieval king, Alfonso X, with ordering that wine should only be served with food. Both stories are told in every bar and neither can be verified.
What is certain is that in several Spanish cities the habit became a custom of hospitality. In Granada and in León, among other places, a drink often arrives with a small free plate. Granada is the best known. A caña brings a tapa, the next round brings a different and usually larger one, and in some bars the third is almost a meal.
How it works
The bars compete, and the standard of the free plate is part of their reputation. Regulars know which counters give hot dishes, which give fried fish and which give small sandwiches. Customers often move from bar to bar through an evening, ordering a drink at each and eating a plate in each, and the evening ends without a formal dinner.
In most of Spain the tapa is now sold separately. The free version survives mostly in a few cities, and local people defend it as part of what makes their bars different.