In a letter when do you use yours faithfully or sincerely?

When you have written a formal letter, it is always important to finish it correctly. If the letter is to someone you have addressed by name at the top of the page, use 'Yours Sincerely', but if you have addressed them as Sir/Madam, or something similar, 'Yours Faithfully', shows a vouch of confidence in the person, which they will appreciate.