Envelopes are a simple product but play a much more integral part in our lives than most people give any credit for. With all the different sizes, shapes, styles and colours that are available, it is hardly surprising it can be confusing.
For a product that is used every day, most people give little thought to envelopes until they run out of them!
Envelopes are a big business despite alternative forms of communication. In fact the Internet, take examples like Amazon and eBay, is actually generating new types of postal traffic, e.g. padded bags, gusset envelopes, book wraps, postal tubes, etc.
Consider the many thousand of envelopes used each day by business and industry for stationery, mail shots, invoices, etc. and you can start to understand.