They’re so ubiquitous that it’s a surprise to learn envelopes were a mid-nineteenth century invention. Somewhere in Gaskell I remember a character protesting at this new-fangled costly creation. Previously people folded the sheet of paper in on itself to form its own envelope, as was replicated later in aerogrammes.
Postage was expensive and so was paper – you would entrust a letter to someone travelling to save postage. Before the creation of stamps, the cost was often paid by the recipient, an interesting notion if you weren’t that enthusiastic about the writer or the contents.
Similarly, to save paper they often wrote across from left to right, then turned the sheet 90 degrees and again wrote across it. Pretty hard going on the reader.
‘Little quarter of sheet notes, without any envelopes – that invention was unknown in those days – but sealed in the corners when folded up instead of gummed as they are fastened at present.’
Source: Elizabeth Gaskell, Wives and Daughters (London: Penguin Classics, 1986 (1866)), p. 387