Letter-writing is an important form of communication that was especially important before modern technology. There are two main types of letters - formal letters which are used for official business, and informal letters which are personal. Formal letters follow a specific format including sender and recipient name/address, date, salutation, reference to prior correspondence if any, content organized into paragraphs, and a complimentary close and signature. Informal letters have a more flexible format but still include sender/recipient name or address, date, salutation or greeting, content, and closing. Both types of letters can provide suggestions but use different language - formal recommends or advises while informal suggests or says to consider an idea.