How envelopes are made

Every envelope is a flat sheet that was folded around a rectangle, and cutting diamonds wastes less paper.

stations: 6
things upstream: 57
raw sources: 18
steps from the ground: 10

The short version

An envelope is a flat paper blank cut to a diamond, cross or kite shape so that folding its sides around a central rectangle forms a pocket. Machines cut the blanks from a reel, fold the flaps, glue the side seams and coat the sealing flap with gum, then count and box the finished envelopes.

Cut the flat blanks

Paper from a reel is guillotined into sheets, and dies cut a blank out of each. Tessellating the shapes across a big sheet, a trick first patented in 1840, leaves almost no scrap.

Goes in
Paper
Punch the window

For window envelopes, a hole is cut in the front so the address on the letter inside shows through. The hole is covered with clear plastic or glassine.
Fold the flaps over

Folding machines have taken over from hand folding since the early twentieth century. They crease each blank and lay the side and bottom flaps over the middle rectangle.
Glue the seams

A thin line of glue holds the overlapping flaps together so the envelope becomes a closed pocket open only at the top.

Goes in
Adhesive
Gum the sealing flap

A stripe of gummed adhesive is applied along the edge of the top flap. The sealing adhesive dries until the sender moistens it or presses it down.

Goes in
Adhesive
Count and box

Finished envelopes are stacked, counted and packed flat in boxes for shipping.

Goes in
Cardboard box

End of the lineEnvelope

Worth repeating

Until 1840, every envelope in the world was cut and folded by hand.
The British mass-production machine of 1845 made envelopes by creasing pre-cut diamond-shaped blanks, which were still folded by hand.
The plastic window in a window envelope can cause problems in paper recycling.