Easy To Open Creative Envelopes

Prior to 1845, hand-made envelopes were all that were available for use, both commercial and domestic. In 1845, Edwin Hill and Warren De La Rue were granted a British patent for the first envelope-making machine.
The "envelopes" produced by the Hill/De La Rue machine were not as we know them today. They were flat diamond, lozenge (or rhombus)-shaped sheets or "blanks" which had been precut to shape before being fed to the machine for creasing and made ready for folding to form a rectangular enclosure. The edges of the overlapping flaps treated with a paste or adhesive and the method of securing the envelope or wrapper was a user choice. The symmetrical flap arrangement meant that it could be held together with a single wax seal at the apex of the topmost flap. (That the flaps of an envelope can be held together by applying a seal at a single point is a classic design feature of an envelope).
Nearly 50 years passed before a commercially successful machine for producing pre-gummed envelopes effectively as we know them today appeared.
The origin of the use of the diamond shape for envelopes is debated. However as an alternative to simply wrapping a sheet of paper around a folded letter or an invitation and sealing the edges, it is a tidy and ostensibly paper-efficient way of producing a rectangular-faced envelope. Where the claim to be paper-efficient fails is a consequence of paper manufacturers normally making paper available in rectangular sheets, because the largest size of envelope that can be realised by cutting out a diamond or any other shape which yields an envelope with symmetrical flaps is smaller than the largest that can be made from that sheet simply by folding.