Letter of introduction
The letter of introduction, along with the visiting card
Visiting card
A visiting card, also known as a calling card, is a small paper card with one's name printed on it. They first appeared in China in the 15th century, and in Europe in the 17th century...

, was an important part of polite social interaction in the 18th and 19th centuries. It remains important in formal situations, such as an ambassador
An ambassador is the highest ranking diplomat who represents a nation and is usually accredited to a foreign sovereign or government, or to an international organization....

 presenting his credentials, and in certain business circles.

In general, a person would not interact socially with others unless they had been properly introduced, in person or by letter. A person of lower social status would request a patron of higher social status to write a letter of introduction to a third party, also of higher social status than the first person, but lower than the second person. It was important to observe the niceties of etiquette
Etiquette is a code of behavior that delineates expectations for social behavior according to contemporary conventional norms within a society, social class, or group...

in requesting, writing and presenting such letters, in such matters as the quality of the paper used, and whether it would be delivered unopened to allow the requesting party to read it. For example, it was best practice to deliver a letter of introduction to the intended recipient with a visiting card, to allow the recipient to reciprocate by calling upon the sender the next day.
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