Here are the main categories of diphthongs:
1. Based on the direction of movement:
* Rising diphthongs: The tongue moves from a lower position to a higher position.
* Falling diphthongs: The tongue moves from a higher position to a lower position.
2. Based on the starting vowel:
* Fronting diphthongs: The first vowel is a front vowel (e.g., /i/, /e/).
* Backing diphthongs: The first vowel is a back vowel (e.g., /u/, /o/).
3. Based on the ending vowel:
* Centring diphthongs: The second vowel is a central vowel (e.g., /ə/).
* Other diphthongs: The second vowel can be any other vowel, including front, back, or central vowels.
Examples of different types of diphthongs:
* Rising, fronting, centring diphthong: /eɪ/ (as in "say")
* Rising, backing, centring diphthong: /oʊ/ (as in "go")
* Falling, fronting, other diphthong: /ai/ (as in "my")
* Falling, backing, other diphthong: /au/ (as in "how")
Note: The classification of diphthongs can vary depending on the specific language and the phonological theory being used.
It's also important to note that some languages, like Spanish and French, have a wider range of diphthongs than others, like English.
In summary, diphthongs can be classified based on the direction of tongue movement, the starting vowel, and the ending vowel. These classifications help us understand the different types of diphthongs that exist in languages around the world.