Where do the tone marks go?

Tone marks in Hanyu Pinyin always go over vowels, not consonants. But even those familiar with Hanyu Pinyin are often uncertain about which in a string of vowels takes the tone mark. If, for example, you are given "huai4" -- is it hùai, huài, or huaì? (Answer: the second choice.)

Fortunately there are no ambiguities to worry about, even where there are several vowels in a row. Various complicated rules explain the placement. Fortunately, in application they boil down to a few very simple guidelines:

A and e trump all other vowels and always take the tone mark. There are no Mandarin syllables in Hanyu Pinyin that contain both a and e.
In the combination ou, o takes the mark.
In all other cases, the final vowel takes the mark.

The possible vowel combinations are listed below, with the vowel that receives the tone marked as second tone.

	a	e	i	o	u
a			ái	áo
e			éi
i	iá, iáo	ié		ió	iú
o					óu
u	uá, uái	ué	uí	uó
ü		üé

Note: Early versions of Hanyu Pinyin also used ê. But since it never was combined with other vowels it is not included here. (It has since been supplanted by ei.)