Although these two variations mean the same thing, there is much confusion around the proper spelling. A catalog(ue) is (1) an itemized list of offerings, and (2) to make an itemized list. The word actually entered the English language as cataloge around the 15th century, a version of the word that is no longer in use. Although American English just recently shifted from catalogue to catalog, there are documented instances of catalog as early as the 16th century. The French-influenced version, catalogue, took hold soon after and by the 17th century this was the preferred English version by most.
Why the change?
In the late 19th century, the U.S. were pushing to develop a uniquely American version of the English language. Catalog was included in this movement and began to be the favored version of the word. A large portion of Canadians also began adopting this version of the word - However, French-Canadians prefer to use catalogue as it coincides more closely with their language.
Which version of the word is proper?
The answer to this question is still unclear - within Canada and the U.S., Catalog is the more utilised version of the word. Outside of North America however, catalogue is the dominant version. British-English and Latin-English languages strictly use the catalogue version. Below you can view the Ngraphs (provided by google) of the two versions compared in American English and British English.
Catalog vs Catalogue - A visual representation
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