What is "tone number"

n. A number allocated to a syllable in various transliteration systems such as Hanyu Pinyin indicating the tone in which it is pronounced.

Tone numbers are numerical digits used like letters to mark the tones of a language. The number is usually placed after a romanized syllable. Tone numbers are defined for a particular language, so they have little meaning between languages.

Other means of indicating tone in romanization include diacritics, tone letters, and orthographic changes to the consonants or vowels. For instance, in Mandarin, the syllable (which has a falling-rising tone) is represented in Wade-Giles romanization as ma, with a tone number; in Hanyu Pinyin as mǎ, with a diacritic; and in Gwoyeu Romatzyh as maa, with a change in the vowel.