Envelopes are the one item most people never really think about. When it comes to envelopes there are many different sizes and terms, just the same as paper. There some common envelopes such as a #10 (the standard business letterhead envelope) and a #9 (the standard reply envelope that comes with all of your bills) and then there are tons of other “standard” sizes such as A2, A7, 4-Bar, #6 3/4 and so on. Have a look at this chart.
