Common coins come in several different denominations based on the relative worth of the metal from which they are made. The three most common coins in the world are the gold piece (gp), the silver piece (sp), and the copper piece (cp). One gold piece is worth ten silver pieces. One silver piece is worth ten copper pieces.
In addition to the common coins, other unusual metals sometimes come to surface in transactions. The electrum piece (ep) and the platinum piece (pp) originate from fallen empires and lost kingdoms, and they arouse suspicion and skepticism when used in transactions (-2 to Bargain checks). An electrum piece is worth five silver pieces, while a platinum piece is worth ten gold pieces.
A standard coin weighs about a third of an ounce, so fifty coins equal a pound. These are sized in such a way as to make this work.
A typical commoner uses copper and silver pieces and sometimes gold. Other coins are mainly for wealthy merchants, nobles, and the hoards of greedy dragons.