Question: what is the difference between a1 and b1 paper size?
Answer: B1 is between A0 and A1 in size, with an area of 0.707 m2 (​1⁄√2 m2). ... B7 is equal to the passport size ID-3 from ISO/IEC 7810. B4, B5 and B6 are used for envelopes that fit C-series envelopes.