In every series of United States coins, there are certain issues with dates or dates with particular mintmarks that are not as plentiful as most other coins in the series. These dates are known as key dates, because in order to complete a collection of that particular series, a collector HAS to obtain those coins. It is highly recommended, that when a collector decides to complete a collection of a particular series of coins, they obtain the key dates first. Once the key dates are securely in place, the rest of the series can be obtained fairly easily and in most cases fairly inexpensively. If a collector decides to collect all of the common date coins first, by the time he has filled that portion of his collection, the key date coins may have risen in price to the point that the collector cannot afford to purchase them. There is no worse feeling for a collector than to know they have almost very coin in a series, but are never going to be able to obtain that last coin or two to complete the collection.