Essays are designs for stamp vignettes, stamp borders, or both combined that were never approved and used in the final printing of a stamp. Even though a design is finally adopted with only slight modifications, it is still an "essay". To attain the status of a proof, a proposed design, or essay, must be exactly like the issued stamp for which it was submitted.
Private bank note companies made all US stamps from 1847 to 1894. Those firms submitted essays and printing bids. As early as 1851, the U.S. Post Office Department established a policy of advertising for stamp proposals to be accompanied by essays, or examples of the stamps to be furnished. After the bids were opened, an Expert Committee was asked to rule on the designs, colors and paper.