An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat objects, such as a letter or card.

Traditional envelopes are made from sheets of paper cut to one of three shapes: a rhombus, a short-arm cross or a kite. These shapes allow to make the envelope structure by folding the sheet sides around a central rectangular area. In this way, a rectangle-faced enclosure is formed with an arrangement of four flaps on the reverse side.


Envelopes are classified according to the following criteria:

  • by size;
  • by the presence of a window;
  • by the location of the closing flap.

International standard sizes

International standard ISO 269 (since withdrawn) defined several standard of envelope sizes, which are designed to use with ISO 216 standard paper sizes:

Format Dimensions (Height × Length) Suitable content format
DL/E65 110 × 220 mm 13 A4
C7/C6 81 × 162 mm 13 A5
C6 114 × 162 mm A6 (or 12 A5 or 14 A4)
C6/C5 114 × 229 mm 13 A4
C5 162 × 229 mm A5 (or 12 A4)
C4 229 × 324 mm A4
C3 324 × 458 mm A3
B6 125 × 176 mm C6
B5 176 × 250 mm C5
B4 250 × 353 mm C4
E4 280 × 400 mm B4

Layout of C6, C6 / C5 and DL envelopes and location of the recipient address window on them

The window in an envelope often made in the form of a rectangle with rounded corners, closed from the inside with a clear film and allows to see the contents partially (for example, the address). The window can be of any size and position, but it is usually located in the lower right corner and measures 90 × 45 mm. The distance from the edge of the window to the edge of the envelope is not less than 15 mm.

By the location of the closing flap

Depending on the location of the closing flap, envelopes are classified into two types:

  • envelopes with a side flap, in which the fold of the flap coincides with the side (short) left edge of the envelope relative to its front side;
  • top-flap envelopes where the fold of the flap aligns with the top (long) edge of the envelope from the front.