When do you put an attention line in a business letter?

You use an 'attention line' when you address the letter to the business name and address. The attention line tells the person who sorts the mail at that business which person or department should get your letter.

An alternative to a separate attention line is:

Ms. Ellen Essential

Marketing Manager

ABC Company

P.O. Box...

Or, if you don't know the person's name:

Marketing Manager

ABC Company

P.O.Box...