A flat paper container, wrapper, or cover, as for a letter or thin package, usually having a gummed flap or other closure method; abbreviated "ep". Envelopes are classified by flap style (ie: flat, square, wallet, pointed, mail point), by opening (ie: open-side , open-end ), by seam (ie: side, center, diagonal), and by dimension (ie: mailable, mailable with surcharge, non-mailable). The most useful ISO envelope sizes are C3 - C6, but a special C6/C5 format was created by DIN 678 to replace the most popular DL business size, which is incompatible with automated postal machines. The DL designation originally meant "DIN Lang", but the abbreviation is now more diplomatically explained as "Dimension Lengthwise". The ISO 269 envelope standard does not include transparent "address window" envelopes, so the international standard is defined by DIN 680. See courtesy envelope, C sizes, converter, pre-consumer waste, kraft paper.