Collectors use catalogs like the United Postal Stationery Society (UPSS) to identify and grade these rare pieces.

They can include airmail designs, different color proofs (such as purple or red), and various paper types.

They are test concepts submitted by companies like the International Envelope Co. or Nesbitt.

These items are highly collectible because they represent the creative and developmental stages of postal history. They often feature unique variations in color, layout, or advertising "collars" that never made it to the general public. Key Characteristics of Essay Envelopes

In philately, an refers to a design for a postage stamp or postal stationery that was submitted to the authorities for consideration but was not officially adopted for use in that exact form.

Some historical essays included "advertising collars"—decorative or promotional borders around the stamp area.