Chinese is a tonal language, and the standard Mandarin variety has four separate tone categories. That means that each meaningful syllable of the language belongs inherently to one or another of these four categories.

Categories are phonemes - that is, calling them categories is a way of classifying them according to an abstract but simple system that does not depend on how they sound. The way they actually sound is as "tone contours", meaning either a level pitch or an upward or downward glide on a vowels or voiced fricative. But it is not necessary to know how they sound to know which words go in which category.

There are many ways to notate these tones in romanization. The widely used Pinyin system uses diacritics. Gwoyeu Romatzyh spells the tones into the words themselves. Wade-Giles and most other systems ignore the tones (as does Pinyin in most non-textbook applications).

Although Unicode can represent the Pinyin diacritics, I prefer to use systems that remain readable in purely ASCII environments. Since most people cannot read Gwoyeu Romatzyh, I and many other people instead use numbers to indicate the four tones of Mandarin. Here are the numbers, together with the traditional names of the tone categories:

  1. the yinping or "upper level" category
  2. the yangping or "lower level" category
  3. the shang or "rising" category
  4. the qu or "departing" category
Note that these names do not correspond in any simple way to the sounds of the tone contours themselves. They are of great antiquity, and appear to have been chosen less for meaning than for the fact that each is an exemplar of the tone category it represents.

For comparison, here are the diacritics used in Pinyin for the same categories:

  1. macron over the main vowel
  2. acute accent over the main vowel
  3. hacek over the main vowel
  4. grave accent over the main vowel
Note that these diacritics do not correspond to their familiar sounds in European languages; they are intended to be roughly iconic (representing an aural feature "directly" by visual means) for the contours of the tones:
  1. macron: high level tone
  2. acute: pitch rising from mid-register to high
  3. hacek: low pitch, dipping in some accents; rises in pitch when syllable is in final position
  4. grave: pitch falling from high to low or mid-low
These are only the tones of standard Mandarin; tones vary enormously even within varieties of Mandarin itself, so it would be silly to try to give a comprehensive accounting of the whole situation in a small place.
But in case you are deeply interested, here are the tone numbers used in Taiwanese. There are actually several sets, but I will describe the two most widely used, together with the "Church romanization" system (one of two important systems). The traditional numbering system goes like this:
  1. the yinping or "upper level" category; represetend with no diacritic
  2. the yinshang or "upper rising" category, represented with an acute accent
  3. the yinqu or "upper departing" category, represented with a grave accent
  4. the yinru or "upper entering" category, represented with p, t, k, or h at the end of the syllable and no diacritic

  5. the yangping or "lower level" category, represented with a circumflex
  6. there is no yangshang or "lower rising" category any more! but it is still assigned a number for etymological reasons; some people use the number for other purposes
  7. the yangqu or "lower departing" category, represented with a macron
  8. the yangru or "lower entering" category, represented with p, t, k, or h at the end of the syllable and what looks like a dotless exclamation point (or a straight apostrophe) over the main vowel
Church romanization also attempts to be iconic in its choice of diacritics, though of course the pitch values are totally different from the Mandarin values. You can see that the "upper" tones are numbered 1-4 and their "lower" correspondents 5-8. The competing system, more common among comparative dialectologists working outside of Taiwan, is:
  1. the yinping or "upper level" category
  2. the yangping or "lower level" category
  3. the yinshang or "upper rising" category
  4. the yangshang or "lower rising" category
  5. the yinqu or "upper departing" category
  6. the yangqu or "lower departing" category
  7. the yinru or "upper entering" category
  8. the yangru or "lower entering" category
Here the "upper" tones are represented by odd numbers and their corresponding "lower" forms are even.