The most common double consonants are 'ss, ll, cc, tt, rr, and pp.' Double consonants can be divided into two categories:

1.1 The largest category is when double consonants stand together without any change in the pronunciation. For example, 'rr' in the word diarrhea sounds /r/ even if this word takes one 'r'. This phenomenon might go back to the written rules of old English. Some linguists believe that in a few decades, double consonants with the same sound will disappear.

1.2 Another group is when two consonants stand together and produce two different sounds, like 'cc'. The reason is that the consonant itself has two main sounds. For example:

Vowels can also stand together to make a double vowel. The most common double vowels are ee and oo. Here is the summary of these two double vowels:

'ee' mainly sounds /i:/ and it can stand at the beginning, in the middle, or at the end of a word:

'ee' also sounds /ɪə/:

'oo' in the middle of words mainly sounds /uː/:

'oo' in the middle of words also sounds short /ʊ/:

'oo' in the middle of words sounds /ʌ/, only in these two words:

'oo' in the middle of words and always before 'r' can also sound /ɔ:/. So 'oor' always sounds /ɔːr/.