What is an avoirdupois ounce?
An avoirdupois ounce is the ordinary ounce of the imperial and US customary systems, as opposed to the troy ounce used for gold and silver.
It is equal to 1/16 of a pound or approximately 28.35 grams. The avoirdupois system is based on a pound of 16 ounces, with larger units such as tons and pounds used for heavier measurements.
What are pennyweights?
Pennyweights are a unit of measurement commonly used in the jewelry industry to measure the weight of precious metals such as gold and silver.
A pennyweight is 1/20th of a troy ounce, which is 1.55517 grams. Jewelers still use it for small amounts of gold and silver.
To get grams from pennyweights multiply by 1.555 – 10 pennyweights is about 15.55g. The abbreviation dwt comes from d for denarius, the old penny.