A4 is the everyday paper size in Taiwan, Europe, and much of Asia. One sheet is 210 millimetres by 297 millimetres. Almost every school printer, homework page, and office letter here is A4. It is the same international A-series as A3: if you cut an A3 sheet in half across the middle, you get two A4 sheets. In the United States the common page is a little different (called letter size), so an American “letter” page is not quite A4 — a little shorter and a little wider. When someone says “print it on A4,” they mean this standard page, not thick paper or special paper. Though actually, people sometimes say “an A4” to mean one sheet, the way we say 「一張A4」.