What Envelope is used for
- C4 / 9 × 12 in: unfolded A4 or Letter
- C5 / 6 × 9 in: sheet folded once
- DL / #10: sheet folded in three
- C6 / A6: postcards and cards
ISO C sizes
A C envelope is sized to take the A sheet with the same number: A4 in C4, A5 (or A4 folded once) in C5, A6 (or A4 folded twice) in C6. DL takes A4 folded in three.
US A-style is not ISO A
US invitation envelopes named A1, A2, A6 and A7 have nothing to do with ISO A paper sizes; the number only identifies the envelope.
Questions
What envelope fits A4 paper?
C4 (229 × 324 mm) unfolded, C5 folded once, DL folded in three.
What size is a legal-size envelope?
Usually 9.5 × 15 in (241.3 × 381 mm), for Legal sheets unfolded. For folded documents, the #10 business envelope (4 1/8 × 9 1/2 in) is standard.
What envelope fits a 4 × 6 card?
An A6 invitation envelope (4 3/4 × 6 1/2 in).