Envelope

An envelope is a common packaging item used to contain flat objects. Most often, people use them to hold letters or cards. They are usually made from thin, flat materials like paper.

The structure of a traditional envelope relies on specific geometric shapes. These shapes include the rhombus, the kite, or the short-arm cross. These shapes allow the sides of the paper to fold around a central rectangular area. This process creates a rectangular-faced enclosure. On the reverse side, the folding creates an arrangement of four flaps.

There are several ways to secure these flaps to keep the contents safe. Most modern envelopes are pasted or gummed together at the overlaps. In some cases, a person might use a wax seal to secure the topmost flap at a single point. Some envelopes include a window, which is a hole cut into the front side. This window allows the paper inside to be seen. This design is useful because the receiving address printed on the letter becomes visible. This saves the sender from having to write the address twice.

Americus F. Callahan designed this windowed envelope in 1901 and patented it in 1902.

Envelopes have evolved through many different forms over thousands of years. The earliest known envelopes date back to 3500 to 3200 BC in the ancient Middle East. These were not made of paper, but were hollow clay spheres. They were molded around financial tokens for use in private transactions. Jacques de Morgan and Roland de Mecquenem discovered these in 1901 and 1907. Much later, paper envelopes were developed in China. Known as chih poh, these were used to store money gifts. During the Southern Song dynasty, the Chinese imperial court used them to distribute money to officials.

Before the industrial age, making envelopes was a slow, manual process. Up until 1840, every envelope was handmade. Workers had to individually cut each one from a rectangular sheet. George Wilson of the United Kingdom changed this in 1840. He patented a method called tessellating, which means tiling many patterns across a large sheet. This method reduced the amount of paper waste produced during cutting. In 1845, Edwin Hill and Warren de la Rue patented a steam-driven machine. This machine could cut, crease, and fold the shapes.

Today, the scale of envelope production is massive. Most of the 400 billion envelopes made worldwide every year are machine-made.

Envelopes also serve many specialized roles in modern society. A return envelope is a smaller, pre-addressed envelope included inside a larger one. These are used for courtesy replies or business mail. Some envelopes are even designed to be reused as the return envelope to save money. The direct mail industry uses these as a way to get responses from customers. Some people even use wallpaper to make envelopes when they face financial hardships, as seen during the U.S. Civil War.

Global standards help organize the many different sizes of envelopes. The International Organization for Standardization once used ISO 269 to define sizes like C4, C5, and C6. While these were withdrawn in 2009, other systems remain. Germany uses the DIN 678 standard. The DL size in Germany comes from the "DIN Lang" or "Long" size from the 1920s. In the United States, there are dozens of sizes. The No. 10 envelope is the standard size for business. The U.S. Postal Service has specific rules for letter pricing. For example, a letter must be at least 0.007 inches thick.

Different cultures also use envelopes for specific social traditions. In Japan, envelopes can be categorized by how they open. Traditional Kaku and Chou envelopes open on the short side. Western-style envelopes, called Yōgata, open on the long side. Envelopes are also used to express emotions in ceremonies. In Japan, white and black cords on an envelope represent death for funerals. Red and silver cords are used for weddings.

These cultural uses show how a simple piece of paper can hold deep social meaning.