The next class up from postcards are letters. Starting at 49¢ they are one of the most common forms of mail used to date. I get the feeling that they’ve stopped teaching people how to write and address a letter in elementary school anymore though, because I have seen some really unfortunate letters before.
What comprises a good letter?
A good letter can weigh no more than 3 ounces. If it weights more than 3 ounces, then you don’t have a letter, you have a flat (see Mailing Flats & Parcels). Just as a general guideline:
1 oz = 5 pieces of paper and a #10 envelope
2 oz = 10 pieces of paper and a #10 envelope
3 oz = 15 pieces of paper and a #10 envelope
A good letter will have both a “to” and “from” address. It is important to have both because if the postal service is unable to deliver the letter to the recipient (for a myriad of reason) they need to know where to send it back to. If they don’t have a return address, or have insufficient information to facilitate a return, they will “kill” the letter (see FAQ’s for “Dead Letters”).
A good letter will be oriented properly. Sorting scanners are programmed to process letter sized envelopes in landscape format and do not function well in portrait format. Regardless of how you address your envelope, they will probably be scanned like they are in landscape format. 
A good letter will have a correctly formatted address. This axiom can be both flexible and restrictive. Much depends upon what you are sending and how you are sending it. For more information on address formatting, check out my post called A good address is a happy address!
A good letter will not have anything solid inside. No paper clips, binder clips, coins, credit cards, small rocks, or apples. Anything solid in the letter can cause an additional handling charge for the recipient to pay before they are able to pick it up, or it may be sent back to you with a note requiring more postage. Sometimes these extra inserts slip through, however, but because of the speed at which they are being sorted (again, see my How the Post Office Works page) coins often pop out of the paper, credit cards can be bent or broken, and paper and binder clips have caused many a jam over time. But they got better 🙂
A good letter will not be stuffed full
