Where should the address go?

Use portrait A4, 210 × 297 mm, measured from the top-left corner. The drawing shows both windows for the same recipient.

A4 sheet measuring 210 by 297 mm with two 85 by 28 mm address windows 42 mm from the top, at 17 mm on the left and 107 mm on the right.

Use portrait A4, 210 × 297 mm, measured from the top-left corner. The drawing shows both windows for the same recipient.
The left window starts 17 mm from the left and 42 mm from the top, measuring 85 × 28 mm. The right starts at 107 mm; text starts at 19 or 109 mm.
Start the first address line 48 mm from the top and finish by 68 mm. Use at most six lines of Helvetica or Arial at 9 to 10 pt. Check a 100% print inside your envelope.