A phoneme is the smallest sound unit in a spoken word, a change in which produces a change in the meaning of the word. For example, the /b/ in box is a phoneme. Sometimes two letters are combined to represent one phoneme, for example /th/ in thought. In this case two letters can be considered as one grapheme. Conversely, it is also possible for one grapheme to represent two phonemes, e.g. the x in box is /k/ /s/.