B4, B5, and B6 are used for envelopes that will hold C-series envelopes. B4 is quite common in printed music sheets. B5 is a relatively common choice for books. B7 is equal to the passport size ID-3 from ISO/IEC 7810. Many posters use B-series paper or a close approximation, such as 50 cm × 70 cm ~ B2.
b4 b5 and b6 are used for envelopes that will hold c-series envelopes b4 is quite common in printed music sheets b5 is a relatively common choice for books b7 is equal to the passport size id-3 from iso/iec 7810 many posters use b-series paper or a close approximation such as 50 cm × 70 cm ~ b2