Spelling in Gwoyeu Romatzyh
The spelling of Gwoyeu Romatzyh (GR) can be divided into its treatment of initials, finals and tones. GR uses contrasting unvoiced/voiced pairs of consonants to represent aspirated and unaspirated initials in Chinese: for example b and p represent IPA [p] and [pʰ].
Source: Wikipedia — Spelling in Gwoyeu Romatzyh (CC BY-SA 4.0)