Usage
The 1857 12¢ was primarily used to pay for heavier domestic mail, multiple rates, or international postage. It was commonly used for mail to Canada or, in combinations, to cover the 24¢ or 30¢ rates to Europe during the period just before the U.S. Civil War

Domestic Mail: It paid the 12-cent rate for a domestic letter weighing between 1.5 and 2 ounces (four times the standard 3-cent half-ounce rate). Pairs or strips were often used to cover even higher registration fees or heavy package rates

International Mail:: It was frequently used for mail traveling to Great Britain and other European destinations. The rate for most European countries was 27¢ so you would frequently see a strip of two 12¢, supplemented by a 1¢ stamp. For Russian the rate was 37¢ and which usually had a strip of three with the additional 1¢ stamp. For England the treaty rate was 24¢ and therefore you will commonly see two individual 12¢ or a pair of the 12¢ stamps on cover.