When it comes to rarities in American numismatics, the discussion often starts with the total original mintage for a particular issue. In most cases, the total quantity minted does provide a fairly good indication of whether or not an issue is rare. As an example, the key date coin for the Lincoln Cent series is the 1909-S VDB Cent, which also had the lowest mintage. However, in some cases the rarest issues of a series cannot be determined by the mintage level alone. Rather the more pertinent factor is the total number of surviving pieces.
The 1927-D Double Eagle is one of those issues whose rarity should be determined by the total number of surviving examples, and not solely by the total mintage. The issue represents one of the rarest Saint Gaudens Double Eagles ever publicly released with only a little over a dozen pieces known to exist. While the mintage for the issue was somewhat low, this is not the primary factor behind its rarity.
The reason for the rarity of this particular double eagle is that virtually none ever left the Mint. In the late 1920s the economy was slowing (which would eventually lead to the Great Depression) and few of the Branch Mint double eagles were needed for circulation. The double eagles struck at the Philadelphia Mint were more than sufficient and often exported in quantity as part of foreign trade, making these issues mostly available today. In the case of many of the Denver and San Francisco minted twenty dollar gold pieces from the period, the pieces remained in storage only to be melted years later. With only a small number of pieces escaping this fate, rare issues were created irrespective of the original mintage levels.