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 . Even this simple rule creates issues requiring special cases ; for example , d is , specifically , an abbreviation of the verb do ; the noun do representing the note of the musical scale is a different word , and must be spelled out .