Twelve dwt means that the piece of silver weighs 12 pennyweight. One pennyweight is equal to 1.555 grams; therefore, a 12-pennyweight silver piece weighs 18.66 grams. This is only a measure of weight, however, and does not indicate the purity of silver.
The d in dwt stands for denarius, the biblical coin similar to a penny. Pennyweight is a unit of mass in troy weight, a system commonly used to weigh precious metals. Other units of troy weight include grains, troy ounces and troy pounds. Troy ounces and pounds are not to be confused with the avoirdupois ounces and pounds more common in everyday use.