What is correct, United States dollar or United States Dollar?
In the examples below the emphasis is mine.
Example 1 (context)
The United States dollar (sign: $; code: USD; also abbreviated US$) is the official currency of the United States of America.
Example 2 (context)
As a result, the United States Dollar was defined as a unit of weight equaling 371 4/16th grains (24.057 grams) of pure silver.
Example 3, in definition of the word cable (context)
9. (finance) The currency pair British Pound against United States Dollar
Is there a general rule about currencies? What is the explanation in grammar terms?