How to Read an IRS Notice or Letter 1. Notices are written in the format CP 2. The number may be found in the document's upper right corner. Three letters are in the Letter 4 or LTR 5 format. The number can be seen at the top or bottom right corner of the envelope. 2. Letters are written in the format CP 53. There is only one C in the word "letter." The first two digits indicate which form the letter comes on and what file number it relates to. For example, a letter dated 0115 shows on its face that it comes from Form 1040 rather than Form 1040-EZ. It also shows that this is case No. XXX-XX-XXXX, which means it is part of file No. XX-XX-XXXXX. 3. Notices can be delivered by mail, e-mail, or in person. Letters must be delivered in person. If you do not deliver a letter to the proper address, another letter will be sent after the initial one is ignored. 4. Notices usually tell you what tax law has changed or what action the IRS is taking with respect to your return. Letters generally deal with specific cases or issues within your tax situation. If you have questions about a notice or letter, call the office that issued it. The phone numbers are listed in most notices or letters sent by postal mail.