For instance, in Mandarin, the syllable 馬 (which has a falling-rising tone) is represented in Wade-Giles romanization as ma3, with a tone number; in Hanyu Pinyin as mǎ, with a diacritic; and in Gwoyeu Romatzyh as maa, with a change in the vowel letter.
These are consistent across all Chinese dialects, reflecting the development of tone diachronically.
In the later stage of Middle Chinese, voiced consonants (such as b-, d-, g-, z-) began to merge into voiceless ones (p-, t-, k-, s-) and such voiceless-voiced consonant contrast was substituted by further high-low pitch contrast (yin, and yang).
Note: Tone sandhi rules and the unstressed syllable of Mandarin are not listed here for simplicity.
Even for Pinyin, tone numbers are used instead when diacritics are not available, as in basic ASCII text.