Most of the world outside North America mails on ISO paper sizes, A4 rather than the U.S. letter format, and envelopes for that system follow their own numbered scale headed by the letter C. The C6, at 114mm by 162mm, is the size built specifically to receive an A4 sheet folded into thirds, functioning as the international counterpart to a North American #10 envelope in both purpose and everyday role. The proportions of the ISO system make the C6 more versatile than its single stated purpose suggests. Because ISO paper sizes are related by simple halving, an A4 sheet folds down to A5, and a C6 envelope happens to accommodate a flat A5 sheet or card just as comfortably as a folded A4 letter. That makes it a natural size for invitations, greeting cards and any printed piece designed at A5 or A6 that needs to travel flat rather than folded, in addition to its main job as a business correspondence envelope. International trade correspondence, European subsidiaries of North American companies, businesses that mail regularly to European or international addresses, and anyone printing stationery meant to pair with A4 letterhead all have reason to stock C6 envelopes rather than a U.S.-standard size. It's worth noting that a C6 is measurably smaller than a #10, so any correspondence produced on U.S. letter paper needs a different fold pattern, or a different envelope, to travel comfortably inside one. As with any envelope, the practical choices are the same regardless of size standard: whether the envelope needs a window for a visible address, what closure style suits the mailing volume (gummed flaps for smaller runs, self-seal for anything mailed in bulk), and what stock weight matches the contents. A slightly heavier or textured stock is a common upgrade for C6 envelopes carrying invitations or greeting cards, where the envelope itself is part of the recipient's first impression rather than purely functional packaging.