Tones

The pīnyīn system also uses diacritics for the four tones of Mandarin, usually above a non-medial vowel. Many books printed in China mix fonts, with vowels and tone marks rendered in a different font than the surrounding text, tending to give such pīnyīn texts a typographically ungainly appearance. This style, most likely rooted in early technical limitations, has led many to believe that pīnyīn's rules call for this practice and also for the use of "ɑ" (with no curl over the top) rather than the standard style of the letter "a" found in most fonts. The official rules of Hànyǔ Pīnyīn, however, specify no such practice. Note that tone marks can also appear on consonants in certain vowelless exclamations.

1. The first tone (Flat or High Level Tone) is represented by a macron (ˉ) added to the pīnyīn vowel: ā (ɑ̄) ē ī ō ū ǖ Ā Ē Ī Ō Ū Ǖ
2. The second tone (Rising or High-Rising Tone) is denoted by an acute accent (ˊ): á (ɑ́) é í ó ú ǘ Á É Í Ó Ú Ǘ
3. The third tone (Falling-Rising or Low Tone) is marked by a caron/háček (ˇ). It is not the rounded breve
(˘), though a breve is sometimes substituted due to font limitations. ǎ (ɑ̌) ě ǐ ǒ ǔ ǚ Ǎ Ě Ǐ Ǒ Ǔ Ǚ
4. The fourth tone (Falling or High-Falling Tone) is represented by a grave accent (ˋ): à (ɑ̀) è ì ò ù ǜ À È Ì Ò Ù Ǜ
The fifth or neutral tone (Neutral Tone) is represented by a normal vowel without any accent mark: a (ɑ) e i o u ü A E I O U Ü
(In some cases, this is also written with a dot before the syllable; for example, ·ma.) These tone marks normally are only used in Mandarin textbooks or in foreign learning texts, but they are essential for correct pronunciation of Mandarin syllables, as exemplified by the following classic example of five characters whose pronunciations differ only in their tones. The words are "mother", "hemp", "horse", "admonish" and a question particle, respectively. Numbers in place of tone marks
Since most computer fonts do not contain the macron or caron accents, a common convention is to add a digit representing the tone to the end of individual syllables. For example, "tóng" (tong with the rising tone) is
written "tong2". The number used for each tone is as the order listed above (except the "fifth tone", which, in
addition to being numbered 5, is also sometimes not numbered or numbered zero, as in ma0 (吗/嗎, an interrogative marker).

Rules for placing the tone mark

The rules for determining on which vowel the tone mark appears are as follows:
If there is more than one vowel and the first vowel is i, u, or ü, then the tone mark appears on the
second vowel.
1. In all other cases, the tone mark appears on the first vowel.
2. (y and w are not considered vowels for these rules.)

The reasoning behind these rules is in the case of diphthongs and triphthongs, i, u, and ü (and their
orthographic equivalents y and w when there is no initial consonant) are considered medial glides rather than
part of the syllable nucleus in Chinese phonology. The rules ensure that the tone mark always appears on the
nucleus of a syllable.
Another algorithm for determining the vowel on which the tone mark appears is as follows:
1. First, look for an "a" or an "e". If either vowel appears, it takes the tone mark. There are no possible
pinyin syllables that contain both an "a" and an "e".
2. If there is no "a" or "e", look for an "ou". If "ou" appears, then the "o" takes the tone mark.
3. If none of the above cases hold, then the last vowel in the syllable takes the tone mark.

The character "ü"
An umlaut is placed over the letter u when it occurs after the initials l and n in order to represent the sound
[y]. This is necessary in order to distinguish the front high rounded vowel in lü (e.g. 驴/驢 donkey) from the
back high rounded vowel in lu (e.g. 炉/爐 oven). Tonal markers are added on top of the umlaut, as in lǘ.
However, the ü is not used in other contexts where it represents a front high rounded vowel, namely after the
letters j, q, x and y. For example, the sound of the word 鱼/魚 (fish) is transcribed in pīnyīn simply as yú, not
as yǘ. This practice is opposed to Wade-Giles, which always uses ü, and Tongyong Pinyin, which always uses yu. Whereas Wade-Giles needs to use the umlaut to distinguish between chü (pinyin ju) and chu (pinyin zhu), this ambiguity cannot arise with pinyin, so the more convenient form ju is used instead of jü. Genuine ambiguities only happen with nu/nü and lu/lü, which are then distinguished by an umlaut diacritic.
Many fonts or output methods do not support an umlaut for ü or cannot place tone marks on top of ü.
Likewise, using ü in input methods is difficult because it is not present as a simple key on many keyboard layouts. For these reasons v is sometimes used instead by convention. Occasionally, uu (double u), u: (u followed by a colon) or U (capital u) is used in its place.

from Wikipedia
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