A pennyweight is an unit of mass which is the same as 24 grains, 1/240th of a troy pound, 1/20th of a troy ounce, approximately 0.055 ounces or approximately 1.555 grams. A penny was literally, as well as monetarily, 1/240th of a troy pound of sterling silver.
The troy pound (lb) and the pennyweight lost their official status in the United Kingdom of Great Britain and Ireland in the Weights and Measures Act of 1878. Only the troy ounce and its decimal subdivisions remained legal after then, and even now in the 21st century the troy ounce enjoys a specific legal exemption from metrication in the UK.
Custom jewellers still use the pennyweight in their calculations for the necessary amount of precious metals in casting items using the lost wax process.
While "dwt" is the most common abbreviation for pennyweight (as "d" was the abbreviation for "penny" in the pre-decimalisation British system), some sources list "pwt" as an alternative.

From:www.wikipedia.org