The need to deliver written messages is as old as writing itself. But envelope was rarely used even after postal services first began in the 17th century in England and 18th century in the U.S.http://www.postalheritage.org.uk/history/keydates/ A letter was simply folded, sealed upon itself with wax or a small disk of adhesive called “wafer” and addressed upon the blank side. When the first postage stamp, the Black Penny, was issued in 1840 in England, the British government offered a prize of 200 pounds for the best prepaid post-wrapper design. The winning design by William Mulready, although known as the Mulready Envelope, was merely a diamond-shaped sheet of paper with a rectangle design in the center ready for folding.http://www.postalmuseum.si.edu/industrywhitepapers/EMA_History.pdf

The Mulready Envelope was considered the grandfather of modern envelope although it was hardly popular and slowly disappeared. But the Postal reform leading to the widespread use of the Black Penny in Britain was soon replicated by other European countries, Brazil and the United States.http://www.bbc.co.uk/dna/h2g2/A1082558 It helped the development of the envelope industry as more businesses and individuals use the affordable mails.http://www.postalmuseum.si.edu/industrywhitepapers/EMA_History.pdf Ready-made envelopes were produced by hand in the early days. But the Industrial Revolution propelled the envelope industry into the age of machines and mass production. The first patented envelope folding machine was invented by Edwin Hill in England in 1845. In America, Dr. Russell L. Hawes patented his invention in 1853.http://www.postalmuseum.si.edu/industrywhitepapers/EMA_History.pdf Hawes’ folding machines increased production to 2,500 envelopes an hour. An ever improving series of envelope folding, gumming, self-gumming and drying machines have appeared since the 19th century and made the production of modern envelopes fully automatic.http://www.postalmuseum.si.edu/resources/6a2o_envelopes.html

The C series envelope sizes were introduced to go with the A series paper sizes covered by the ISO 216 standard. The A0 size has an area of one square meter and an aspect ratio of the square root of two. Each subsequent size is half of the previous one when folded parallel to the shorter lengths.http://www.papersizes.org/ The C series envelopes match with the corresponding A series paper sizes. For instance, a C4 envelope holds an A4 sheet of paper unfolded whereas a C5 envelope holds an A5 sheet of paper unfolded and an A4 sheet of paper folded in half.http://www.papersizes.org/c-envelope-sizes.htm