American invitation sizes are named after the envelopes they go into, which is why a "5 × 7" and an "A7" are the same thing and neither has anything to do with ISO A7. Once you know that, the family is simple and every piece of a wedding suite has an obvious slot.
THE ONE THING TO REMEMBER
US A7 = a 5 × 7 inch card. ISO A7 = 74 × 105 mm, smaller than a business card. If you're briefing across the Atlantic, give millimetres or inches — never the letter code alone.
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US ANNOUNCEMENT CARD FAMILY · TO SCALE 1.1 PX PER MM · BASELINE ALIGNED
A1 3.5×4.875 89×124
RSVP · 4-bar
A2 4.25×5.5 108×140
note · thank-you
A6 4.5×6.25 114×159
details card
A7 5 × 7 in 127 × 178 mm
THE invitation size
A9 5.5×8.5 in 140×216 mm
program · menu
Fig. 5 — The announcement family to scale. Each card takes the envelope of the same name. Postcards are the one format where a government agency sets your artboard. In the US, three thresholds matter: the minimum size a card can be mailed at all, the maximum size that still qualifies for the cheaper postcard rate, and the point at which it becomes a letter or a flat.
A 32 pt cotton card is far too thick for the postcard rate — that's a letter-rate piece regardless of its dimensions. Worth knowing before you print 500 of them and take them to the counter.