Rather than going through the hassle of opening the dictionary dialog box via the Tools menu, you can add terms directly from the document window: If you have typed a word that is flagged as a spelling error, simply right click on the word and select Add to Dictionary from the shortcut menu. The word will be added to your custom dictionary. If you havent created a custom dictionary, a new one will be created for you (the default name for dictionaries created with this method is CUSTOM.dic).
More Word Processing Quick Tips

