Envelopes of various sizes
The coding for envelope sizes follows that of the paper sizes. A bundle of A4 sheets of paper will fit into a C4 envelope. A C5 envelope will take a bundle of A4 sheets that have been folded into halves. An A4 sheet folded twice, into quarters, will fit into a C6 envelope, as will a C6 greetings card, or some postcards.
Envelopes also come in various styles. Greetings card envelopes normally have a diagonal, and gummed, flap. Their paper weight is frequently 80gsm. Correspondence cards need a straight flap and a substantial weight of paper, normally 100 or 120gsm. Office envelopes come in all sorts. Traditionally