Representing phonemes

The writing systems of some languages are based on the phonemic principle of having one letter (or combination of letters) per phoneme and vice-versa. Ideally, speakers can correctly write whatever they can say, and can correctly read anything that is written. (In practice, this ideal is never realized.) However in English, different spellings can be used for the same phoneme (e.g., rude and food have the same vowel sounds), and the same letter (or combination of letters) can represent different phonemes (e.g., the "th" consonant sounds of thin and this are different). In order to avoid this confusion based on orthography, phonologists represent phonemes by writing them between two slashes: " / / " (but without the quotes). On the other hand, the actual sounds are enclosed by square brackets: " [ ] " (again, without quotes). While the letters between slashes may be based on spelling conventions, the letters between square brackets are usually the International Phonetic Alphabet (IPA) or some other phonetic transcription system.