The escudo was a gold coin that had a value of 16 reales, the equivalent to two, eight real coins. If printed in America today it would be the equivalent of a two dollar coin. The escudo was first minted in 1535. It was also minted as a double escudo worth 32 reales. A double escudo was commonly called a doubloon or a ducat in England. There was also a 4 escudo coin as well as an 8 escudo coin. Just like in the real each escudo was an 1/8th of an ounce. The 8 Escudo coin was called the 'Onza' or ounce.

It is worth noting that an ounce of silver or gold in the 1600's could buy a lot more than a U.S. dollar can today. A single 8 reales coin would have the buying power today of approximately $100.00 or approximately $12.50 per real. A doubloon would have the buying power of $400.00 not accounting for golds current trading value. It is clear to see why piracy was so lucrative.
Some coins, particularly those originating in the new world were often poorly formed and struck. These were called cobs. Cobs were generally made at the mine and not in a mint. A chunk of silver or gold could quickly be weighed, hammered flat and struck with a handheld stamp and hammer. They would then be shaved down to make sure the weight was perfect (or to skim a little if the minter were dishonest). This often left the coin looking misshapen. Because they were carefully weighed and carried an approved stamp they were generally accepted as legitimate currency.

When coins would be brought to another country they would often be tested for weight and by hammering a mark into the coin to verify that it was solid. This was called a 'chop mark' and each assayer had his own mark, a sort of seal of authenticity. This is also why in older movies people would bite a gold coin.