Tone letters are sets of symbols assigned to individual tones of a language in order to help with correct phonology and enunciation. The use of tone letters is particularly important when writing in a language with many spoken tonal nuances. Words in some languages are spelled exactly the same but can have quite different meanings when the speaker simply alters the tone of one syllable. The symbols used for tone letters are classified according to tones that rise or fall in pitch, and these sets of signs are normally called tone contours. Several Asian and some African languages use tone letter and number systems for clarity in written words.
The basic contour tones of a language are usually designated as rising or falling. One of these sounds that descends in pitch before rising is called a dipped tone, and one that rises before falling is known as a peaking tone. These changes in pitch generally need to be long enough in sound duration to be placed in these categories. Shorter and more abrupt changes in pitch are usually called stopped or clipped tones. Words with clipped tones are normally spoken with a specific enunciation technique known as a plosive stop.
Symbols used to designate each of the tone letter categories vary depending on the specific language. The Mandarin language of China has a five-pitch range with each pitch assigned its own tone contour symbol. Tone letters for the two highest pitches are typically written as two long vertical bars per tone with a short horizontal dash at the top of one bar and the bottom of the other. The middle three pitches of this rich tonal language are written as vertical lines with dashes at various points along them. Most other languages with tone contours also have five-pitch ranges with variations in sound according to each language's unique phonology.
Standard typed text presents some challenges for written tone letters because conventional computer keyboards do not have letter keys assigned to these symbols. Many word processing software programs also have limited numbers of these symbols that can be inserted into text. Tone numbers provide a convenient solution as symbol substitutions, as most languages have variations in the pitches corresponding to each number. Some Chinese tonal ranges have the lowest numbers assigned to the lowest tone, and some African ones have the opposite values assigned to ranges of high to low tones.