In the field of American numismatics, assembling a complete set of a series can be extremely challenging. In this case, a “complete” set is defined as containing all date/mintmark combinations. With only a few exceptions, virtually every American coinage series contains one or more rare and expensive dates. For instance, take the 1908-1929 $5 Indian series. Most of the dates can be had for $500-$1000 each, but two dates (the 1909-O and 1929) are both five-figure coins. A respectable 1929 can cost more than a dozen common dates!
A more extreme example is the 1854-1889 $3 Princess series. The “stopper” date is the unique 1870-S; the sole known specimen is currently impounded in a museum. Despite having been damaged, the coin sold for $687,500 in 1982 and is worth multiple millions today. Thus it is currently impossible for a new collector to assemble a complete set of $3 gold pieces.
To circumvent these prohibitively expensive dates, many collectors instead choose to form year sets instead of complete sets. That is, instead of buying every date and mintmark combination, the collector instead buys one example of every year. So, going back to the $3 gold series again, one could use an 1870 $3 (worth $2000-$5000 in average grades) for that year rather than buying both the 1870 and ultra-expensive 1870-S.
Another option is to form a decade set. This is a popular option for series that are filled with pricey dates. The 1840-1907 $2.50 Liberty Quarter Eagle is an extremely long and challenging series; even date sets are a major effort to complete. This is a perfect series to be collected by decade, as there are affordable dates in each of the seven decades.
Yet another option is to form a mintmark set. The 1839-1908 Liberty Half Eagle series lends itself especially well to this approach. A complete set is virtually impossible; the 1875 is a six figure coin and just one 1854-S is known (it is valued in the millions). Even assembling a year set is a daunting task that would take a great deal of patience and money. However, the $5 Liberty Half Eagle has the distinction of being the only U.S. coin struck at seven mints: Philadelphia, Charlotte, Carson City, Dahlonega, Denver, New Orleans and San Francisco. A mintmark set would include one representative from each mint. It would cost the same or potentially less than a decade set—and dramatically less than a year or complete set.