In English, there are 23 words which are replaced with a single letter. That way, the word "but" is contracted (shortened) to the single letter "b," "can" to "c", "do" to "d", and so on. Even this simple rule has exceptions and special cases, which must be thought of. As an example, only the verb "to do" is replaced by "d", the noun "do" (which stands for a note in music) is a different word, and is written in full.