Originally designed most likely as a way to extend the use of precious timber, pergolas—going back at least as long as the Egyptians, say 3000 years give or take—were in their earliest forms coverings with spaced slats. Over these slats were grown vines or fruit trees. These covered seating areas would used in the more temperate regions as a cool respite for the sedentary pursuit of watching the wildlife from your back porch, de rigueur for the noble class of 17th century England.
Here, paradise and cocktails under the logs:
If their design began as a practical matter we've so long ago grown enamored by their sheer object beauty of the things that their essential function hardly matters anymore. We just love to look at them. Most of the great gardens have at least one if not more.
A stone cold rush of rustic with a mixer of Mediterranean:
Wine and pyramids under the shady green:
Detached from the natural garden and incorporated into the paved one, pergolas are often used as a poolside shelter.
The word pergola, originally from the Latin, literally refered to a "covered eave." It is part of a tradition that traveled the world. Moroccan style, below:
It's been argued that it was the Italians during the Renaissance who first incorporated the shelters as free standing elements in their formal gardens. In Italian pergola means "a close walk of boughs."
Here, a constructed forest featuring roses and waterlilies.
As interpreted by Andrea Cochran:
Cloth will work nicely, too.
A more finished rendition from architect Daniel J. Lieberman:
Small is also good.
The room without walls: