The earliest examples of the Cape Cod line of Imperial Glass that were commercially available were produced in 1932. To see if a piece dates to this period, check the trademark, which is usually located on the bottom. A strong light is an important tool for this process, as trademarks are sometimes hard to distinguish. If the piece is from the earliest days of the Cape Cod design -- 1931 through 1950 -- it will have a double letter "I" along with the word "Imperial."
Pieces from the middle period of the Cape Cod line will have a different trademark. Beginning in 1951, the trademark symbol became an "I" placed directly on top of a "G." To see if a piece dates to this period, check the bottom of the piece for this superimposition of letters.
The last period of Cape Cod design -- the period just before the factory closed its doors due to bankruptcy in 1984 -- bears yet another trademark design. To see if a piece dates to this last period, check the piece for an "N" superimposed over an "I" -- meant to represent "New Imperial."