The courier or messenger bag is distinguished from other leather briefcases and bags by the size, shape and style. They usually have a flap from the top of the back over the main compartment to one-half or full coverage on the front. This flap provides protection and a place to put another zippered pocket or to attach smaller compartments to organize and store smaller gadgets. They can be a horizontal or vertical format and be as small as a man bag or document case; or as large as a laptop case or satchel – such as a mail bag.
In my opinion, more important issues lie in determining the history of the two bags and explaining their usefulness. Couriers or messengers have been deployed in times to war to carry important documents from one commander to another. Courier bags have consisted of little more than a leather pouch with a pocket large enough to hold paper sized documents, and possibly contained a map pocket or plastic window to protect the map in adverse weather conditions (in more modern applications – WWII and on).
Another type of messenger case has been developed by those supplying electrical and telephone line men. They have used a shoulder bag with a flap, from one-half to full overlap. The bags have been made of canvas, leather or a combination of the two materials. They were used to carry tools and heavy metal fasteners up the poles, while keeping the line man’s hands free for climbing and working.
Postal carriers have used a large leather bag fashioned in the same configuration. The Pony Express even used a leather messenger bag so the weight of the mail could be evenly distributed for a comfortable ride, protect the mail from rain and dust, and be handed-off quickly. Bicycle couriers in the large cities have also used this style and format to dispatch their mail and documents around the major cities.