English orthography is the alphabetic spelling system used by the English language. Like other alphabetic orthographies, it uses a set of habits to represent speech sounds in writing. In most other languages, these habits are regular enough so that they may be called rules. In standard English spelling, however, nearly every sound is spelled in more than one way, and most spellings and all letters can be pronounced in more than one way and often in many different ways. This is partly due to the complex history of the English language,but mainly because no systematic spelling reform has been implemented in English, contrary to the situation in most other languages.
Take for example the letter sequence OUGH. It is by far the sequence of letters with the most unpredictable pronunciation, having at least six pronunciations in North American English and over ten in British English. A few of the more common are these:
“though” (cf. toe)
“through” (cf. true).
“rough” (cf. ruffian).
“cough” (cf. coffin).
“thought” (cf. taut).
“bough” (cf. to bow).
“hiccough” (cf. to hiccup)
Source: Wikipedia