Double-vowels 双母音--ai, ei, ao, ou

A double-vowel (compound vowel) is a vowel that is composed of two single-vowels. There are 4 double-vowels: ai, ei, ao & ou.
 
Pronouncing Tips

• The shape of mouth and the position of the tongue are changed from the first single-vowel to the second-single vowel.
• The first single-vowel is the main vowel; while as the second singe-vowel is only a tail sound. The tone mark is placed on the first single-vowel.
  

Double-vowel: ai

Pronunciation starts with the <a> and the tip of the tongue is resting on the lower gums. Then, the blade of the tongue glides toward its position for <i> and stops at the mid-high position. 

Double-vowel: ei

Pronunciation starts with the tongue position for <e>, and glides toward the position for <i>. The tongue starts in a position,  which is slightly forward and raised and stops in the mid-high position.

Double-vowel: ao

Pronunciation starts with the tongue position for <a>, the blade of tongue rises toward the position for <u>, but stops at the position for <u>. The <o>position in the pinyin for <ao> was originally <u>.

Double-vowel: ai

Pronunciation starts with the tongue position for <o>, the blade of tongue rises toward the position for <u>, but stops at the position for <u>. At the same time, the lips slightly purse, causing the mouth to become partially rounded.