The traditional account* states that ㅂ b is derived from ㅁ m by the addition of a stroke (ㅁ而ㅂ […] 其因聲加畫). However, Gari Ledyard proposes instead that ㅂ b is borrowed from Phagspa b, ultimately from Tibetan བ b, and that it is ㅁ m which is derived by the removal of the top of ㅂ b.
* Hunmin Jeongeum Haerye "Explanations and Examples of the Proper Sounds for the Instruction of the People" (1446), defining and explaining the script now known as 한글 (Hangeul, “Great script, Korean script”) in South Korea and 조선글 (joseon'geul, “Korean script”) in North Korea.
- 비읍 (bieup), a jamo (letter) of the alphabet of the Korean writing system, hangeul; the unaspirated bilabial plosive (IPA: /p/)
Usage notes 
In the North Korean order, ㅂ (b) is the sixth jamo. In the South Korean order, it is the eighth.